To enter upon a discussion of the manner in which plateaus and mountains were formed, would make it necessary to resort to such judgments as we could draw from their external appearance and their internal structure. The rapid progress of geology does indeed afford us many probabilities thoroughly grounded. A few of these may have been briefly indicated in connection with some elevated regions, where the massiveness is striking, and where the axis of elevation is prolonged to a considerable extent. In such cases the influence exerted on the world is more evident than it could be elsewhere.
Alexander von Humboldt has employed the term Intumescence, to indicate the manner in which plateaus have been upheaved. Plateaus appear as long, often wide, mostly level, sometimes rolling, sometimes hilly elevations, presenting an appearance as if the earth had swelled with confined gases, and with depressions here and there as if,in the casting of the molten mass within, a natural external subsidence had followed. They have, therefore, viewed in their internal structure, an unbroken wholeness, and are free from those vast fissures which characterize mountains, rending the earth for hundreds of feet down. The utmost want of uniformity is seen in the gradual depressions which often harbor the large internal lakes found in great plateaus. Varied as they are in configuration, they always retain marks enough to indicate that they owe their upheaval to steady, gentle, and not tumultuous forces within, exerted at the time of the primeval cooling of the earth’s crust; in contrast, therefore, with mountains, which were thrust up from beneath, through huge seams made by the bursting through of pent-up vapor and gases. These elevations of the earth’s crust, whether in the form of mountain or plateau, must correspond, in order that the symmetry of the globe may be preserved, to the depressions found in lowlands and beneath the water of oceans and seas.
It is observable that the great plateau upheaval of the Old World has taken the shape of a belt, which runs in a northeasterly direction along its whole southeastern shore, crossing the equator at an angle of 45°, broken, however, at some places, but never so much as to destroy the coherence of the belt. The diagonal of the rhomboidal plateau of eastern Asia, passing due northeast through the table-land of Thibet, indicates the direction of the whole band of highlands. This band drops toward the south in uniformly steep declivities; while toward the north it falls away with gradual steps of transition, reaching at length the regions of the greatest depression—Libya, northern Arabia, the Caspian, Siberia, and, at last, the low regions around the north pole.
In this belt or chaplet of plateaus lie the high table-lands of South and Northeast Africa, Abyssinia, South Arabia, Persia, Beloochistan, North Deccan, Afghanistan, Thibet, East Tangut, and eastern Gobi, in Mantchooria.
Correspondent with this immense plateau belt, in the New World, is the great American chain, once a wholly volcanic, and though differing so much in structure, direction, and hydrographical influence, yet giving the globe a wholeness, a unity in diversity, which is strikingly apparent.
The linear regions of elevations of the earth’s surface, as we may term them, in contradistinction to the plateaus which are characterized by breadth rather than by length, have been projected in the form of mountain chains, as has been already hinted, through huge fissures made by the rending of the earth’s crust. The upheaval to fill the seam has, in some cases, been all made at once; in others, in a succession of periods. The uniform agreement of all the geological strata or their diversity decides this point. Sometimes the rocky strata are laid bare and easily investigated. Often, however, the observer is obliged to draw conclusions from a part to the whole. Yet in all cases the mountain, in contradistinction to the adjacent plateau, is the tract which has been thrust through the crust. The frequent steep and lofty precipices show the immensity of the internal force required to lift the mountains from their places, while the lines of stratification indicate the direction of upheaval. The rifting of a seam in the earth’s crust was the first step in the formation of mountains; the filling up of the seam by liquid matter, the second step. The upheaval of Asia, from the Persian plateau to Gobi, in a line 60° N. E., seems to be connectedwith the most ancient revolution which the earth’s crust ever experienced. The mountains there are, therefore, more modern in origin than the plateau on which they stand. The direction of the chain, in all cases, seems to have been dependent on the direction of the fissure in the earth’s crust, which the mountain range afterward fills. The breaking through the crust necessarily occurred when the pressure beneath the surface was very great, or when a moderate pressure was exerted beneath a thin crust, where the resistance was slight.
The latter case seems to have been prevalent in most plateau regions. Their own gradual upheaval probably thinned the surface, and made it more liable to fracture. This accounts for the fact that the greatest mountains of the globe are found contiguous to plateaus. And the broader the original seam in the crust was, the broader the mountain range which rose to fill it, either at a single upheaval, or in a series of convulsive throes projecting successive masses of molten matter from below. In the latter cases the strata thus formed lie on each other like the leaves of a book, their constitution changing according as the more advanced stages of melting in the vast internal caldron throw out more metamorphosed rocks. These later layers rose to a greater or less height on the sides of the partially-formed mountain, according to their specific gravity, their more or less fluid state, and their rapidity of cooling, as we can now see by examining the layers in their present permanent condition.
Thus far we can conjecture, with great security, taught by the manifestly wild and fierce convulsions which once threw up the mountains, since in them distortion is the rule and regularity of structure the exception, and also by the equally manifest quiet and sustained process of upheaval,when the plateaus were formed; their strata being in a state of regularity and unbroken repose.
When the great vents produced by the outward pressure of internal volcanic forces occurred beneath the sea, they were filled up in the same manner as on the dry land, excepting that the summits of the mountains emerge above the surface in the form of isolated islands, or when there was a chain or group of mountains upheaved, as an archipelago. When there was no rifting of the surface, and no forcing up of whole chains of peaks through a thinned crust, the fierce action of the internal heat appears to have necessitated the upheaval of solitary volcanoes here and there, in some cases even rows of them, to give vent to the pent-up steam and gases, and to convey away the molten tide within. When such volcanic series rose in parallel ranges, they lifted, or may have lifted up the whole district between them, as if upon their shoulders, and so formed the American type of plateaus, of less breadth and greater length than the Asiatic, and in height corresponding with the volcanic peaks which form their rim, and to which they are probably indebted for the form of their structure.
It needs hardly to be added to what has been said above, that the general direction of existing mountain chains depends upon the direction of the primitive seams made in the earth’s surface by internal forces. The Ural Mountains, the Scandinavian chain, the Alleghanies, the Ghauts, run on meridian lines; others more or less transversely.
The various kinds of rock which have been thrown up in mountains enlighten us as to the process and results of the internal heat of the earth; the successive formations display not only the various eruptions of molten matter, andits discharge in new layers above what had been thrown out before, but reveal the relative age of the various formations. We have in a single chain sometimes a whole volume of history, marking off the epochs of upheaval with the most perfect legibility and exactness. Many crystallised rocks result evidently from the gradual process of cooling after the ancient exposure to the intense heat of the inner portions of the earth—granite, porphyry, gneiss, slates, and the so-called metamorphosed rocks. These used to be considered the oldest formations, but the upheaval theory treats them as the latest formed.
Most mountain chains have been uplifted to their present height by a succession of upheavals. To accomplish this has been labor of uncounted thousands of years. Only a very few—the main Carpathian range, for instance—seem to have been upheaved at a single convulsion, and to have assumed their present appearance at once. Where there were incessant eruptions accompanied with flames, and masses of molten matter (lava) have been ejected from the crest or from single summits, these volcanoes and volcanic ranges have been the result. Elsewhere no such phenomena have been visible. Possibly, in such cases, the masses cooled so rapidly as to extinguish or fill up what may have been embryo craters, and the plutonic acclivities may have been repressed, leaving us the traces of primeval eruptions, but no vestiges of any dangerous forces remaining till now. Of mountains formed in this manner, may be mentioned the Puys de Dome, the Bohemian basaltic peaks, trachyte Transylvania Alps, the Katak Kaumene, (“Burnt Tract,”) of Asia Minor, and Hauran, Iceland, parts of the great American chain, parts of the Sunda chain, the South Sea Islands, and Bagdoola, and its range of extinct volcanoes in the Thian-Shan chain.
But other forces besides fire were competent to form mountains and plateaus, to spread layers of clay and sand and various deposits at the bottom of the sea, afterward to harden into strata of rock. In contradistinction to plutonic formations, these have been called neptunic, because formed at the bottom of the sea. The oldest of the neptunic or stratified rocks have been upheaved by the subterranean forces, and now are found in the elevated plateaus or mountain ranges, still having, however, their unbroken irregularity of structure. Also, after the stratification has been complete, and plutonic acclivities have opened the seams in the earth of which I have already spoken, and molten masses have rushed up to fill them, fragments of the primitive stratified rocks have been caught up and raised, together with the molten masses, to the very summits of lofty mountains; so that the geologist finds fossils there more or less perfectly preserved, the stratified rocks which contain them surrounded by the plutonic rock upheaved from below the surface. Chalk layers full of mollusca and infusoria have been found by Humboldt and von Buch on the very summits of the Andes, and corresponding with those which have been discovered by Ehrenburg in the deposits at the bottom of the sea.
Other older and more recent oceanic deposits are found in their primitive condition at the bottom of the sea, or in very low places on the land. In such localities the surface of the earth is composed of horizontal or slightly inclined layers or strata, of secondary formation, and whose origin in deposits from water cannot be denied. These are the beds of chalk, clay, sand, marl, gypsum, and other common substances; and these strata again have been overlaid with more recent accumulations, the result of diluviumor alluvium, continuing even up to the present time.[5]
This variety of the earth’s surface stands in the strongest contrast with mountain regions, or, in one word, with the highland form in all its modifications. The name lowland we apply to all those broad tracts which do not rise more than four hundred feet above the level of the sea. The absolute elevation is determined from a section drawn vertically from the superior surface to the plane of the sea. Every comparison by numbers of one lowland plain with its more elevated surroundings gives only a relative result, as for instance, in comparing the valleys of one chain of mountains with those of a more lofty chain. Such relative lowlands may lie at a great elevation above the sea, as the vale of Chamouni, for example, at the north foot of Mont Blanc, is 3000 feet above the ocean level. Both conceptions of the word lowland, which is common to elevated plains as well as those at the sea’s margin, are entirely different, and should be kept distinct, although they are very often confounded.
We are to deal here only with the absolute, great, and generally diffused lowlands, in contrast with which the elevated valleys and plains just referred to may be considered as mountain table-lands and the rims of plateaus.
We assume, as we did in judging of the two grades of plateaus, an arbitrary standard of measurement, and limit the rise of real lowlands to an altitude of 500 feet above the level of the sea. Great tracts of running plain, rising by so slight a grade as to be almost imperceptible, can be regarded only relatively as lowland, and, in a strict sense, belong to those regions of transition which fall more truly within the domain of highland or plateau. The word plain indicates the opposite of hill or mountain, but has nothing to do with the greater or less degree of absolute elevation, although it is often used as if it had.
The lower limits of lowlands are sharply defined enough. They are the margin of the sea, toward which the slope usually becomes almost imperceptibly small. Often the expression is used, yet not quite fitly, that the lowland extends into the sea for some distance, and is found beneath the surface. Strictly this is the bottom of the sea, and does not fall under consideration in this connection.
Many lowland plains rise so slightly above the sea level, that they are not unfrequently submerged, and, in many cases, owe their existence to repeated overflows. They are the basins of old gulfs, as in the very slightly elevated plains of Caracas, whose whole shore is open to the influences of the great Atlantic current flowing from east to west; or, as in the great Lombardy plain, which slopes at the same almost imperceptible degree toward the Adriatic. There are also some lowlands found in the interior of continents, and these, too, sinking below the level of the sea; but they are altogether exceptional, and only met with intwo or three instances. They are called, by an accommodation of an algebraic term,negativelowlands. To them belong the region around the Caspian and the Aral Seas, and the much smaller tract comprising the Dead Sea, and forming the Jordan valley; besides, there is the Suez steppe, inclosing the bitter lakes between Asia and Africa; and possibly the Beled-el-Jereed, in the western part of the Sahara and the central part of Australia.
To these it might not be incorrect to join those partial lowlands which have been rescued by human efforts from the sea; the marshes, for instance, behind the dikes of Holland, Sleswick, East Friesland, and at the mouths of the Vistula, the Weser, the Nile, the Ganges, and other rivers.
The most extensive lowlands in the world are probably those which embrace Siberia, in Asia, and the Canadian and polar region of North America. Many great tracts, entirely inland, are in those flat districts covered by sea-water which was once driven in by great storms, and now lies stagnant, resulting in inapproachable swamps and morasses. Yet, under the equator, there are immense lowlands, as, for instance, in the eastern Sahara, although this region is broken by strips of plateau, and is by no means that uniform lowland plain which it used to be regarded. Northern Australia belongs to the same category, and also those immense plains which reach from the Atlantic so far into the interior of Brazil, along the lower Amazon. By the time, however, that they reach the middle course of that river, they have acquired, though in such imperceptible steps, a considerable degree of elevation, according to Humboldt’s barometrical observations, and not reckoning certain limestone hills found there, from 1050 to 1200 feet. The plains of the middle Marañon are, therefore, trueplains, but not absolute lowlands, and not to be identified with the great flat region at the mouth of the river, and in comparison with the real lowlands of Venezuela, which do not rise over 200 feet above the sea, and genuine plateau, which, level as it is and broad as it is, is far more elevated than the Valdai plateau, in Russia.
Almost all great river mouths are true lowlands—the Egyptian delta, the delta of the Ganges and the Indus, for instance, (the two latter being separated by the very moderate plateau (100 feet) between Delhi and Mooltan;) to these we may add the delta of the Euphrates, the east shore of China, between the Blue and the Yellow Rivers, and Senegambia, between the Senegal and the Gambia. And in America, the same thing occurs in the Mississippi, Orinoco, Amazon, and La Plata, where the immense mass of water which they send to the sea passes through lowlands of very great extent. In the Mississippi they extend from the mouth as far north as the confluence of the Missouri and the Mississippi, where stands St. Louis, not 500 feet above the level of the sea. The prairies west of the lower course of the river rise rapidly, though imperceptibly to the eye, to the high terraces of Kansas, at Council Grove, varying from 1500 to 2000 feet absolute elevation, and then more rapidly toward the west, to mountain plains or plateaus, from 3000 to 6000 feet high. These, of course, lose the distinctive character of lowland.
The mouth of the St. Lawrence is, in some respects, analogous. Lowlands accompany it for a great distance from the sea; at Lake Ontario the elevation is only 232 feet, at Lake Erie only 565 feet. Yet the level tract is narrowed down to a mere border, and does not widen into great lowland plains. The contracted region of lowcountry along the St. Lawrence is broken up, too, by rocky heights and rib-like ledges, whose absolute height, however, is not to be confounded with the elevation of the plain which they traverse.
In entire contrast are the broad plains of South America, which lie along the course of the Orinoco, La Plata, and Amazon, the so-called pampas and savannas, which extend a great distance into the interior, farther, indeed, than investigators have yet thoroughly prosecuted their researches. In no continent are the distinctions between highland and lowland so sharply drawn as in America. The lowland plains occupy four-fifths of all the country east of the Andes, in South America: only one-fifth is highland; for, notwithstanding the extent of low plateaus and diminutive mountains scattered through these great plains, yet their entire amount is inconsiderable, compared with the immense lowland tracts of that continent. America has fitly been called the region of the greatest depression on the globe, because this is the prevailing characteristic of its whole eastern side, lowlands forming two-thirds of all America, and highlands only one-third.
In Asia, the later hypsometrical observations have shown that the lowlands are by no means so extensive as they were formerly supposed. The highland extends, according to von Middendorf, much farther northeast of the Yenisei, toward the northern limit of Siberia and Tschatschi, than was formerly supposed; and the Siberian plain extending westward to the Ural Mountains is narrowed down from 4,079,970 to 2,233,800 square miles. Yet this lowland comprises, including central Bokhara or Toorkistan, 1,051,200 square miles, and other low Asiatic plains 1,314,000, the enormous area of 4,599,000, or more thantwice the extent of Europe, leaving 9,636,000 square miles for the highlands.
In Africa there are almost no lowlands to speak of, excepting the districts around the mouths of the great rivers indicated a few pages back. To all equatorial Africa this physical feature is entirely wanting. In the north, where the whole Sahara was formerly thought to be one vast low plain, there are now known to be the moderate plateaus already indicated. The area of true lowland is, therefore, sensibly diminished. Vogel’s barometrical observations have already shown us that the country around Lake Tchad is about 1200 feet above the sea; the surface of Lake Tchad is 850 feet above the ocean level, and the lower limit of that region does not, therefore, come within the range already set as the point where lowlands become highlands.
In Australia the lowland seems to be the prevailing physical form, although here and there exceptions to it occur.
In Europe there are three great lowland plains to be specially mentioned. The greatest, that of middle Europe, embraces the shores of the North Sea and the Baltic far inland, and extend the farthest to the southeast. A second, hardly of less extensive proportions, comprises all northern Russia as far as the White Sea and the Arctic. It embraces but one-third of the great polar plain, and is really one with the region beyond the Ural chain. The third is the region around the Black and Caspian Seas.
The Germanic-Sarmatia-Russian plain extends, without a break, from the mouths of the Rhine, through all central Europe, to the middle Volga and the Ural. It is pre-eminentlya region of lowlands, without any elevations of importance, and having no change of level, except very gently undulating swells, and on the north and south margin plateaus which very seldom rise over 500 feet. It begins with the deltas of the Rhine and the Scheldt, in Holland, passes through Lower Westphalia, Lower Saxony, the Marks, Lower Silesia, Lower Gallicia, and Poland, as far as the upper Dnieper and the middle Volga. It extends up the Rhine as far as Strasbourg, 474 feet above the sea, up the Weser as far as Cassel, 486 feet, and up the Elbe as far as Dresden, 280 feet.
The true Rhine delta may be defined as lying between Amsterdam, on the sea, and Dusseldorf, 107 feet above the sea level. Then passing by the broken and romantic tract lying between Dusseldorf or Cologne and Mayence, we come to the true Rhenish lowland, 240 feet above the sea. Munster is 400 feet above the ocean level. East of the Weser is the Lüneburg Heath, which advances in elevation, as we go toward the Elbe and the Havel, to 300 or 400 feet. Brunswick lies at an altitude of 200 feet; Magdeburg, of 128 feet. The height gradually increases; at Wittenburg it reaches 204 feet; at Dresden 280 feet, where the Elbe issues from the highlands; and in Lower Silesia we find Breslau, 375 feet above the sea, and its observatory, standing on the hills around the city, at a height of 453 feet, which seems to be the highest point in the whole vast tract.
Between the Rhine delta and the now dry basin of Paderborn, from the Ems to the Weser, Aller, and middle Elbe, is the mountain tract of the Hartz, (with the Brocken at the north, 3500 feet high,) running up as far as 52½° N. lat. By this natural feature the breadth of the great plain is considerably curtailed. As it is also more tothe east of the Leipsic basin, from which the Mulde, Elbe, and Elster flow, by the hill country of Lausatia and North Silesia, with the Riesengeberge, (Giant Mountains,) 5000 feet high, which extends northward as far as 51° N. lat.
A third basin is in the Silesian, from which the Oder flows toward the northwest, and enters the southern limits of the great plain near Oppeln and Brieg. A third tract of hill country lies on the east bank of the Oder, and extends to the middle Vistula, the Tarnowitz Heights, in Upper Silesia, about 1000 feet in altitude. The plateau north of the Carpathian range, on which Cracow lies, is 669 feet above the sea; and the most northern hill group of Kielce, between the Pilica and the Vistula, rises in the Kreutzberg to a height of 1920 feet, and in St. Catherine to 2000 feet.
The great lowland advances eastward, with always diminishing breadth from north to south, over the extensive plains of the middle Vistula, at Warsaw, 330 feet above the sea; over the Lithuanian morasses of the Bug; over the Sarmatian district of Minsk and Pinsk as far as Kiev, on the middle Dnieper, at the southeast, and as far as Orsha and Smolensk, at the northeast. Pinsk, in the middle of this tract, lies about 400 feet above the sea. The north side of the plain is bounded by the very moderate plateau south of the Valdai hills, at Smolensk, 792 feet high; at Osmana, southeast of Minsk, 882 feet. On the south side it is bounded by the equally moderate plateau of Wolhynia and Podolia, whose absolute altitude is yet undetermined, but which, at the source of the Bug, is about 1000 feet.
This is the great Lithuan-Sarmatian plain, which, east of the Dnieper, is transformed into the central Russian lowland, at whose middle point is Moscow, whose exactelevation above the sea is between 300 and 400 feet; at Kazan, on the Volga, the height above the ocean level is but 270 feet, measuring from the highest point on the banks. Southward, the plain reaches to Simbeersk, 181 feet in altitude. The maximum breadth of this whole vast lowland tract is about 500 miles; the distance between Smolensk and Kiev, and the distance from the central point of the great Russian section to any sea, is between 500 and 600 miles.
The slight elevation of the lowland just described, rising but very little above the sea level, bears, throughout the most of its extent between the dunes of the north and the hill chains of the south, the character of a formation rescued from the domain of the sea within the very latest geological periods. The almost unbroken uniformity of the surface from the Scheldt to the Volga, about 2500 miles, confirms the character which its geological structure indicates. The deposition of disconnected, superimposed layers, running to a great depth, is exactly similar to that which we know results from the action now going on at the bottom of shallow seas. And in the great central European plain there is no sharply-defined geological limit met at the border of the North and the Baltic Seas. The same features extend beneath the surface of both of those seas. This whole lowland is, therefore, to be regarded as an immense basin, now dry, but once the bottom of a great sea,—an extension of the seas which now form a part of its northern border. The old coasts are now seen far inland. Wherever this coast-line changed its course, the whole landscape now alters its appearance; and yet more striking than the external view is the internal constitutionof the soil. Masses of stone, standing out in full view, reveal the inner structure of what lies concealed. And these rocky projections are precisely analogous to the jagged outlines of our present bold sea-shores. The land is not cut up by inlets hollowed out by the action of waves and currents to a considerable depth, yet traces of such movements, and of the physical formations effected by them, are found. Promontories and islands are now found in plateaus, and hills encompassing dry basins. To the latter belong the intervale of the Rhine, and the basins of Paderborn, Leipsic, and Silesia. To the former belong the hills and plateaus of Middle Germany; of the Westphalian Mark, from Elberfeld to Dortmund, or, as might be said, from the Ruhr to the Lippe; the Yeutoburg Forest to the Weser; then the Weser Mountains, and the Hartz to the middle Elbe; the Thuringian Forest and the Ertz Mountains around the Leipsic basin to the upper Elbe; the Lausatian Mountains and the Riesengeberge to the Glatz Mountains, on the upper Oder; the Trebnitz Heights of Silesia, and the lower plateaus of the Fore Carpathian range, embracing Cracow as far as the hills of Kielce and the confluence of the Sau with the Vistula. Along the southern border of the ever-broadening plain are the plateaus of Gallicia, about 1000 feet in height, of Wolhynia and Podolia, and then less elevated plateaus, till we reach the Dnieper.
The geological character of the border of the sea which once covered what is now central Europe, is full of interest, because from it can be deduced all that we can know of the history of those great changes.[6]But we must passover this, and only indicate the geographical configuration of the dry basin as it exists now, and forms the great Germanic-Sarmatia-Russian plain.
In the course of previous remarks on the lowest range of plateaus, I have remarked, that along the south coast of the Baltic the moderately elevated hill chains of Pomerania and of Old Prussia separated the true coast with its lowland from the great interior plain, forming a barrier, averaging about 300 feet in height, with here and there a form which runs hard upon the lower limit of plateaus of the second class; at any rate, a transition from between the lowland and the plateau.
It may here be remarked that the long, low chains, made up mainly of loose sand and other mixed and uncombined materials, and running along the southern border of that long, low band which skirts the Baltic, seem to be dunes once running along the shore of a sea which has now receded many miles to the north. In the deep channels and old inlets now dry, as for instance in the great break through which the Vistula passes below Thorn, only loose breccia, and no united layers of stone, appear. Yet this does not seem to be the case everywhere, although in the Cis-Ural and Baltic depressions dune-like ridges are to be found, some of them rising to a height much greater than was formerly suspected. These, it is true, are scattered, and only partially prevalent, but here and there they ascend to an altitude of nearly 1000 feet. At the eastern end of the great Pomeranian sea-plain west of Dantzic, and between that city and Bütow, where the sand ridge, which formed the ancient shore-line, runs very far to the north, there are a number of villages 400 feet above the sea. The Lower Mountain, (Thurmberg,) 54° 13′ 29″ N. lat., rises to a height of 1024 feet; the hill near Upper Buschkau, eastof the Thurmberg, is 814 feet high; the hill near Hulterfeld, 846 feet; and the Höckerberg, near Schönberg, 902 feet.
Of the Thurmberg, Humboldt remarks that it is the most remarkable elevation between the Hartz and the Ural Mountains, and that but a few points in the Valdai range can be brought into comparison with it. Its position close by the sea is especially noteworthy. It is very probable, according to Humboldt’s opinion, that those inequalities of surface, formed of sand once partly or wholly submerged,—found in Mecklenberg, Pomerania, East Prussia Proper,—and now divided into flats and hill ranges, do not belong to the dune system of the ancient shore-line, but have the reason of their existence in ordinary upheaval; in the formation of limestone, and of the usual Jurassic rocks, which, afterward, have been covered with sand and other loose materials. The peculiar accumulation of genuine marine fossils indicates the existence of upheaved rocks below the upper layer of sand.
It is these elevations which in the constantly advancing ridge or ridges run northeasterly, and take the form of plateaus, increasing in breadth from the water-shed north of Smolensk, and the source of the Dnieper, in the Valdai Forest, and the western Uwalli, and which are found between the Volga and the Dwiner, and thence run eastward as far as Perm, on the Kama. They form the line of demarkation between the great Central European plain and the North Russian lowland, which extends as far up as the Arctic. This easterly chain, so far as it has been measured, seems to be less in altitude than the Valdai hills, which are about 1000 feet high. In East Prussia Proper and Livonia there are elevations of more than 600 feet; about 55 miles south of Dorpat Munnamäggi, thepoint of culmination, according to Struve, is 996 feet above the sea; south of Vilna the heights of Puzewitch reach an elevation of 990 feet.
In the same direction, still northeast, runs the Valdai, forming the source of a number of large streams and the great water-shed of eastern Europe. On the road from St. Petersburg to Moscow, Humboldt found the altitude at Norwaja Ijetza 660 feet, and the highest point at Popowa Gora 792 feet, (according to Pausner, 876 feet.) One point going south from the Valdai, at Mosti Derewna, the latter naturalist has ascertained to be 1032 feet above the sea; and the highest point in the range is, according to Helmersen, 1098 feet. Still further eastward, between the Valdai hills and the lake region between Lake Seligher and Bielo Ozero, the range of uplands, known as Uwalli, running northwesterly, intersected by numerous canals, and forming the water-shed of a number of rivers, gradually diminishes in height, but, still advancing eastward, it rises again, in the neighborhood of Perm and the Kama, to 1014 feet,—about the elevation of the Valdai range. Uwalli is only the Sclavic name of such hills as those whose absolute height is insignificant, but which, crossing as they do the great plains of Poland, Lithuania, and Russia, were formerly confounded with mountain ranges, and were so represented on the maps. They have, of course, great hydrographical value, and play a leading part as the water-shed of eastern Europe.
This second vast lowland is the direct continuation of the central European lowland, with a decided sinking toward the Black and the Caspian Seas, indicated by thecourse of the rivers of that region. It extends from the month of the Danube over the lower Dniester, Bog, Dnieper, Don, and Volga, as far eastward as the Sea of Aral. To the last named the Siberian plain gradually declines. The southern plain of Europe stands in unbroken connection, so far as its formation is concerned, with the West Siberian plain, (2,213,400 square miles in extent,) and is, therefore, one of the most extensive lowlands on the globe. The Baltic-Sarmatian plain is separated from the West Siberian merely by the long Ural chain, (from 50° to 67° N. lat.,) whose elevation is only from 4000 to 5000 feet, and whose breadth is unimportant. Take away the Ural, and a continuous line could be drawn from Breda, near the confluence of the Meuse, Rhine, and Scheldt, across Europe and Asia, following the line of 50° N. lat as far as the Chinese frontier, passing over a continuous series of low, insignificant hills, heathlands, and steppes, and traversing a space estimated by Humboldt to be three times the length of the Amazon!
Toward the south, the Cis-Ural, European side of the Ponto-Caspian lowland, is separated from the Black Sea by a ridge of granite knolls, which passes from Volhynia and Podolia eastward as far as the cataracts of the Dnieper, and thence southeast, with diminished breadth, reaching its limit at Taganrog, on the lower Don, and the Sea of Azof. This ridge separates the narrow steppes of the northern shore of the Black Sea from the lowland of South Russia, the fruitful district of Ukraine. The height of these hills in the west, where they appear to have the greatest elevation, has been estimated to be about 1000 feet above the sea. Toward the Dnieper they have not yet been carefully measured; but probably there they do not rise above 700 feet.
The small sand steppe south of these granite hills runs from the Crimea eastward as far as the North Caucasian steppe, between the Don, Volga, and the Caspian, and indeed may be traced to the northeast as far as the Bashkiric-Ural chain. Lakes of marked saltness are found there: Elton, for instance, which lies 24 feet above the sea; while farther eastward they are found, as for example on the Kamysh and at Samara at a depression of—138 feet, 60 feet below the level of the Caspian Sea. Yet this lacks confirmation.
From this lowland, only a few elevations arise, and these of insignificant absolute height; yet, on account of the extreme uniformity of the whole country, they are objects of amazement to the whole steppe world. The Little Bogdo, south of Lake Elton, and yet farther south, Great Bogdo, 504 feet above the sea, according to Humboldt, and Mount Arsargar, 331 feet in absolute height, according to Murchison, are the only important hills. The Great Bogdo is composed of calcareous limestone and of sandstone, with rich deposits of salt.
The Kirgheez steppe separates, by a plain of very moderate elevation, the north Siberian lowland from the Caspian-Ural depression. It was formerly supposed, and indeed represented on the map, that a mountain range passes through this district from the Ural chain to the Altai. The Kirgheez steppe appears to range from 780 to 960 feet in elevation; while the Siberian plain is but 280 feet above the sea at Omsk, 192 feet at Tora, and 108 feet at Tobolsk. It has been considered by some that the Kirgheez steppe, as well as the granite hills of southern Russia, belong to an undeveloped system of mountains, an early cooling having solidified them before reaching the elevation which they would have attained; and that theypartake of the direction which analogy would teach us such a chain would have, from northeast to southwest, parallel with the Carpathian and the Caucasus ranges.
The great depression of the Old World begins with the deepening of the Volga basin below Simbeersk; and at the place (51¾° N. lat., near Orenboorg and Saratov) where it breaks through the last row of hills in the Obstshei-Syrtis, it commences a rapid descent toward the Caspian and the Aral Seas. This great concavity, on the confines of Europe and Asia, at the center of the greatest land-mass, and far removed from any ocean, is remarkable as having no parallel on the globe. Humboldt remarks that perhaps a similar phenomenon would be repeated at the interior of other continents, if the tertiary formation and the parts found by marine deposit did not exist. It would be profitable to follow out so weighty a thought, with the surface as it now is.
The Obstshei-Syrtis is the moderate range of hills which runs westward in two branches from the Bashkiric-Ural, at Orenboorg, the northern spur running by Uralsk and the Ural River; the southern by Samara, rising on the east shore of the Volga to a height of 600 feet, and ending at Sarepta.
Orenboorg, on the Obstshei-Syrtis, where it leaves the Ural chain, is 255 feet above the sea. Uralsk lies somewhat lower, being 234 feet above the sea. The surface of the Volga, where it breaks through the high banks of Saratov, is only 36 feet above the ocean level; while the western shore, above Saratov, is 562 feet in height. Farther down the river, Sarepta lies 30 feet below the sea level; and there is, therefore, between Saratov and Sarepta, a distance of about 180 miles, a fall of 66 feet. West of the Volga, and following the river, is the continuation ofthe Obstshei-Syrtis, ranging in elevation from 562 feet down to 168 feet. At Sarepta, the low hills which thus far have skirted the Volga turn to the southwest, to the Manitsh steppe, sinking to an elevation of but 75 feet, and extending as far as the Sea of Azof. At Sarepta, too, the Volga turns from its normal southerly course, and strikes southeasterly across the Astracan steppe, entering the Caspian at the City of Astracan, 72 feet below the level of the sea. The level of the sea is 4 feet below the shore on which Astracan is built.
The old statements that the level of the Caspian is 300 feet below the ocean, rested solely on conjectures made by the naturalist Pallas. The influence of this great depression on the warmer climate of that region, the peculiar vegetation of the salt steppes, and the salt morasses which exist where the land is perfectly level, as well as the great beds of oyster-shells and other crustaceous remains, led him to the hypothesis that the whole neighboring district is the dry and deserted bed of a former sea, now shrunk to the comparatively insignificant dimensions of the Caspian. The broken line of bold bluffs which bounds the Obstshei-Syrtis on the south seemed to him to be the northern boundary of this inland sea, into which the Volga entered below the pass of Kamyschin and Saratov. Parrot and Engelhardt supposed that their barometrical elevations in 1811 confirmed Pallas’ theory, that the Caspian lies 300 feet below the ocean. Many hypotheses were based upon their observations; but the whole were at length brought into discredit by Humboldt, who distrusted the accuracy of instruments made at that time. Nothing but a trigonometrical survey from Taganrog to Astracan could give conclusive results, and this was accomplished in 1837, under the auspices of the Russian government. The resultproved that, so far from being 300 or 350 feet below the ocean, the Caspian is not 100 feet. Its depression, as already stated, is about 76 feet.
The level of the Aral Sea, which is evidently closely linked to the Caspian, has not yet been determined with absolute certainty. Barometrical observations were instituted for this end by the expedition under General Berg, which explored that region in the winter of 1826, but the cold was severe, and the results are questionable. The result of their investigations was, however, that the surface of the Aral lies 110 feet higher than that of the Caspian. This would make the Aral to be 34 feet above the sea level. More careful inquiries may, however, determine the level of the two seas to be the same; but at present we have to be content with the results of the expedition referred to, and accept its elevation as 34 feet above the level of the ocean.
Without, however, going into details respecting the Aral, the region around the Caspian and directly connected with it, which is below the ocean level, embraces an area of not less than 131,400 square miles. This survey extends from the Volga to the Ural River, thence to the Emba and the northernmost point of the Sea of Aral, and thence to the salt lakes of Aksakal-Barbi, lying to the northeast of this sea. The tracing of this line from the higher to the lower stages of depression gives clear indications, in the nature of the soil, of the existence of a great sea once occupying that whole tract.
Thus much for the configuration of the Caspian lowland. If to these 131,400 square miles be added the 153,000, or, according to Humboldt, 164,000 square miles of the Caspian itself, the entire depression embraces almost 318,000 square miles, and is greater than France,greater than Germany, and only to be compared with the whole Austrian empire! If to this great region be added the district around the Aral, which sea alone covers nearly 25,000 square miles, and then to this the yet unmeasured surface covered by seas yet to the eastward, the entire region of depression is immensely increased. And then if to this be added the great Siberian plain, whose level is not greatly above the sea, the combined district would be at least once and a half as great as all Europe.
Thinking of the immense extent of this depressed region, whose entire surface occupies no inconsiderable fraction of the interior of the Old World, and whose greatest depth at the bottom of the Caspian is from 500 to 600 feet below the level of the ocean, and looking at it as a phenomenon wholly unique, the question arises: How would such a condition be possible, contradicting, as it seems, all analogies? The answer, could we reach it, would not fail to illustrate many recondite geological questions, and to be full of instruction.
Yet the time has not come when a full answer can be given to this inquiry. We have not yet learned the elementary conditions of this remarkable fact; there are innumerable investigations yet to be made, before we can feel perfectly certain that its reason is understood. Still, there have been some preparatory inquiries entered upon, and some preliminary steps taken toward reaching a conclusion, or, at least, toward assuming a reasonable hypothesis. We have already indicated our belief that this depression is connected with a ring of plateaus which have been upheaved around it, and which now inclose it and isolate it from the ocean.
The hollow has its greatest depth near the southern extremity of the Caspian, where it rises abruptly to the Persian plateau. There pass, in the form of a half circle, the loftiest mountains and plateaus of central Asia. On the west side the Caucasus rises, with its giant peaks of Kasbek and Elbrooz, 15,000 to 17,000 feet high, bearing all the marks of volcanic origin,—avalanches of solidified lava on the sides, a lake lying in the abyss of an extinct crater, and the like.
At the southwest, the Armenian plateau follows the course of the Aras from its mouth back to the huge dome of Ararat, 14,656 feet high. The entire geological appearance of that region—the old lava streams, the trachyte rocks—indicate with equal clearness, as in the Caucasus, the agency of volcanic forces in the upheaval of that district. Traces of this great power are also seen in the caldron-shaped hollows, and in the narrow and deep defiles, which are abundant in that region.
South of the Caspian, which in its southern part reaches a depth ranging as low as 420, 480, and 600 feet, and, according to Hanway, even 2700 feet, rise sharply from the sea the Persian Coast Mountains. The plateau of Teheran, 3400 feet in elevation, is directly beyond, from which rise the volcanic peaks of Demavend, 20,000 feet, and Euczan, 6600 feet high. The Coast chain embraces the Elboorz Mountains, uniformly more than 5000 feet high, but which, at Schemrum, northwest of Teheran, rise to a height of 8560 feet; at Churchurah, southwest of Demavend, to 7650 feet; at Nevo, southeast of Demavend, to 8540 feet; at Nejoster, in the Seriakush, east of Demavend, to 7200 feet; and which, above Asterabad, rise in the Shahkush and the Sundukkush to a height of 7270 feet, and almost everywhere display in their trachyte rocksthe traces of ancient volcanic activity. Still farther to the east, the chain which has girdled the Caspian sinks from the lofty height of the Northern Taurus to 1872 feet; in Meshed, 2628 feet; in Herat, an average elevation of 3400 to 4000 feet. But east of Herat it rises abruptly to the lofty plateaus of Bamian and Cabool, 7000 and 8000 feet high, and in the peaks of Colubeba 16,800 feet. The Hindoo Koosh, at Dsellalabad, rises to a height of 18,984 feet; the table-lands of the Bolor, at the Issikul, are at an elevation of 14,664 feet; while the gigantic Pameer is not yet measured, though its noted Pass is estimated at 18,000 feet above the sea.
At this point we reach almost the 40th degree of N. lat., whence northward the mountain ranges gradually decline in height, after throwing off eastward the great chain of Thian-Shan. From the sharp angle formed by the Hindoo Koosh and the Bolor, where the head-waters of the Gihon rise, that large but commercially unimportant river takes its way westward through the Bokharan table-land, falling so rapidly in its course to Bokhara that at the city its surface is but 1116 feet above the sea, then striking northwesterly to the Aral and Caspian. The course of that stream indicates, therefore, the direction and degree of the mountain slope toward the great depression east of the Caucasus and Armenia, north of the Persian highland, and west of the Hindoo Koosh and the Bolor systems.
The lower course of the Gihon, from Bokhara downward, is through masses of mud, sand, and gravel, and can very easily be conceived to have changed its course in the lapse of centuries, from the Caspian to the Aral, as the course of the Sihon seems also to have changed. The great Bokharan plain is covered in this part with a deposit of dried mud; it is a steppe formed evidently from a nowdry sea-basin, which, no less than the northern shores of the Caspian and the Aral, displays the traces of the oceanic character of entire regions.
Halley, the astronomer, made an attempt to solve the mysterious origin of this great sunken basin, and attributed it to the stroke of a first-class comet! Arago, instead of calling into the scene meteorological forces little known, contented himself, in his theory of its origin, with the forces which we know are active even now on the earth, the plutonic powers which are only half confined by the surface of the globe. No one, he says, will hesitate now to accept the upheaval theory, through which geology is able so clearly to indicate the forces and progress of structure of the soil and rocks. The upheaval of great masses in one place predisposes the depression of districts in their neighborhood, to make good the true relation of highland to lowland. And in this case a compensation may be found, according to Arago, for the great semicircle of mountains which passes around the southern margin of the Caspian basin, in the depression caused by the natural falling in of the adjacent region when the great mountains of western Asia were upheaved.
In longitudinal mountain chains the parallel ranges of valleys have a similar origin; in volcanic chains, which have been thrown up in a circular form, similar depressions have been found in the middle, although, it must be confessed, on a much smaller scale of dimensions than in the Caspian hollow. The same feature is observable in the upheavals, by Von Buch, as observed in the Island of Palma, one of the Canaries, or in the Val di Bove, near Etna. Such depressions would at once fill with water, if connected with the sea, as in the cup-shaped island Santorini, or remain land-locked, if they occur in the interior,as in the case of Lake Laach, as the half-ring of mountains girding the lower portion of the Caspian seems to consist mainly of trachyte thrown up by volcanic agency: the analogy just drawn does not seem too remote. Yet the process of structure must have had other concomitant conditions to account for the vast reach northward of the Caspian depression. It is clear that any such volcanic convulsion as would throw up those vast mountain ranges at the south, must have affected largely the geological condition of all the adjacent region; the extent vertically of this effect would be best ascertained perhaps by deep boring. Unquestionably there was many a revolution in the upper portions of the earth’s crust during the formation of the great Caspian hollow, before it assumed its present condition. From the agencies at work in connection with a great internal ocean, the upper soil, as we have it to-day, was formed.
The Aral and the Caspian Seas remain as the lowest places of that great depression, water being found in them, while elsewhere it has entirely disappeared by evaporation: leaving us broad, low plains, instead of that great ocean which once extended from Persia over all Siberia, and west of the Caspian to the Sea of Azof. A more thorough account, geographical as well as geological, cannot be given till after much more extended investigations have been made into the physical characteristics of this region than as yet have been prosecuted. It may be remarked here, that the waters of the Aral and Caspian are bitter and salt, but not so much as those of the ocean; the bottom is covered with slime and sand. The Aral has a depth ranging from 90 to 222 feet; the Caspian, beginning with its extensive, shallows at the north, deepens toward the south, till, reaching the lower third, its depth is over 600 feet;and thence southward it is no less deep, till it reaches the bold shore of Eusellis. From this lowest point the upheaval begins, which culminates in the great mountains on its southern border.
According to Humboldt’s view, the great Caspian hollow embraces not only the basin of the sea, but a vast dry plain, extending northward as far as Saratov and the Obstshei-Syrtis; even Uralsk lies lower than the level of the Black Sea. The same physical feature, though on a less extended scale than here, is found in Holland, China, Lower Egypt, and Palestine. Subsequently to the emergence of the continents, long before the filling in of huge fissures by mountain chains, and during the continuance of those great convulsions which reach back into the remotest geological periods, the surface of the continents must have been subjected to frequent partial changes of level. The surface undulated probably in that same wave movement which is now observable, though in much less degree, in those earthquakes and upheavals which the whole western part of South America is experiencing even now.
The depressions which have assumed a permanent form since the convulsions which formed them, have gradually filled with deposited soil, and, were the naturalist able to lay bare the primitive rock, he would discover that it exists in the shape of great concavities, without a trace of that evenness which now characterizes the surface. Eichwald has made it probable, by his personal observations, that the upheaval of Ararat and of the Armenian plateau on which this trachyte mountain rests, has driven the Caspian Sea back east of the flat steppe of Karabagh and Mogan, on the lower Aras, to the neighborhood of Bakoo. The water of that sea formerly extended to the confluenceof the Bargashad, (called also Bergershat and Bergernet,) with the Aras, below Ireben. The traces of volcanic action there are decisively evident: the Caspian reached, before that action occurred, up what is now the valley of the Aras, as far as Ararat; and in many places south of Erivan—at Saliyan, in Shirwan, and elsewhere—salt beds of the most crystalline quality, forming whole mountains and whole belts of salt lakes at the confluence of the Aras and the Koor, demonstrate their formation in a former sea which once covered that region. The very recent upheaval of the Ural chain cannot fail also to have had an influence in contracting the dimensions of the Caspian hollow.
Only two kindred depressions to this remain to be spoken of, which, though of not so great superficial dimensions, are of yet greater depth—the depression of the Jordan Valley and the bitter salt lakes on the Isthmus of Suez. These we must consider before we pass from the contrasts between highlands and lowlands to the transitions between them.
The nearest relationship to the Caspian hollow, displayed by any similar feature, is found near the heart of the Old World, in the comparatively diminutive and isolated valley of the Jordan, including the Dead Sea, whose absolute depth below the level of the ocean has been determined only within the most recent period. Many former travelers had noticed, in the deep gulf which holds the Dead Sea, and especially at its north end, near Jericho, a much greater degree of heat than elsewhere in Palestine, and the existence of many plants and fruits whichthey had met in the hotter climates of Arabia and India. The tree which yields the Mecca balsam flourishes in the oasis of Jericho; the product of the balsam of Palestine supplied the pin-money of Cleopatra. A number of German and English observers endeavored to solve the question of the depth of the Jordan basin—von Schubert, Russeger, von Wildenbruch, Moore, and Bake, later Symonds, and Lynch; de Berton and Russeger made the first barometrical observations at the Dead Sea, but they did not attempt to give more definite limits to their results than to assert that its surface is somewhere between 500 and 1100 feet beneath the ocean level.
Von Shubert’s barometer did not suffice to determine this point, but he ascertained the surface of Lake Tiberias to be 535 feet below the surface of the Mediterranean. All barometrical measurements were unreliable at that depth; yet it could not be denied that the depression could not be an insignificant one. A measurement with the level made by Symonds, an Englishman, from Jaffa to the Dead Sea, in 1843, gave us our first sure results. The surface of the lake lies 1231 feet beneath the level of the Mediterranean at Jaffa. The subsequent expedition of the Americans—Lynch, Dale, and Anderson, in 1848—has given the following additional results:—
Soundings of the Dead Sea, made with the greatest care, determined the depth to be, according to Lynch, 1227 feet; according to Symonds, 1970 feet. The entire depression below the ocean level would be, then, by Lynch’s measurement, 1235 + 1227 = 2462 feet; according to Symonds’, 1235 + 1970 = 3205 feet. This is thegreatest known depression on the globe. Jerusalem lies 2449 feet above the ocean level. From the roofs of the city to the surface of the Dead Sea is, therefore, 2449 + 1235 = 3684 feet; and the entire descent from the capital to the bottom of the Dead Sea is 4911 feet, if we adopt Lynch’s measurement, and 5654, if we follow Symonds’.
The basin of the Dead Sea consists of two very different parts—the larger and deeper northern one, the smaller and shallow southern one; the two being separated by a sandy peninsula—el Mesraa—and connected by a narrow channel of insignificant depth. The northern basin seems to owe its present form to the unchanged primitive depression; the southern one to a partial upheaval at some later epoch. But in breadth they do not vary much one from the other; both have their larger axis coincident with the Jordan valley, which here widens a little, but which is still hemmed in here, as farther north, by the parallel ranges of mountains. The chain east of the sea appears to rise a thousand feet higher than the one west of it. The depths of the two basins are entirely unlike. The southern is nowhere more than 12 feet deep, and diminishes to 5 feet and less than this near the shores, so that the southern half of it is entirely unnavigable by craft of any size; and those who wish to land have to wade for a long distance through mud as deep as their ankles. The northern part, on the contrary, attains a uniform depth of more than 1000 feet, from the north to the south; in the northern third it is even 1227 feet; toward the west coast it shoals to between 600 and 800 feet, but is 500 feet deep hard-by the coast. There is but a very narrow rim of shallow water on the western side, and the navigation is, therefore, tolerably safe. On the eastern shore the coast is even bolder, and the descent to deep water immediate.Close by the romantic mouth of the Arnon, embouching through rocks, the depth of the sea is about 1052 feet. So great a difference in the depth of the two basins seems to indicate a considerable diversity in the manner of their formation.
Volcanic activities have been felt in the Jordan valley up to the present time. They manifest themselves in various forms—deposits of salt, hot springs and naphtha springs, asphaltum beds, sulphur fumes, currents of heated air, clouds of smoke, and rumblings beneath the surface. The Jordan valley remained, from Lake Tiberias down, unfilled, as we should infer from analogy that it would be by the upheaval of a chain of volcanic mountains; or by the expansion of an internal lake or sea, the waters accumulating till at last they should acquire such volume as to break away and form new channels. In case the obstructions were too great, they would remain inland lakes. And such is the Dead Sea, its southern border being too high to allow it a free exit into the Red Sea.
Many other fissures or hollows on the surface of the continents would be regarded as lowlands, were they not filled with water. The bottoms of such lakes often sink suddenly to a great depth, while others are lagoon-like, or shallow seas of an entirely different hydrographical character. Internal lakes, regarded as isolated lowlands, merely filled with water, are an especially interesting theme of study; yet much remains to be investigated regarding their structure and historical formation. The Dead Sea has been regarded, up to this time, as the deepest of all such lakes. The greatest depth of the Caspian has not yet been fully ascertained; but if Hanway’s soundings, 2700 feet, are to be relied on, it is very great. Lake Baikal, in its deepest part, between the two steep walls ofrock which rise high above the surface, has not yet been carefully sounded; but as its surface is 1500 feet above the sea, its bottom does not probably fall below the ocean level. The great chain of North American lakes, whose area embraces about 109,500 square miles, are surrounded by level country from 500 to 600 feet above the sea—a region which, in part, falls under the designation of plateaus of the lowest class, and which, in part, comes under the name of lowland; the surface of Lake Superior being 627 feet above the sea, Lakes Michigan and Huron 578 feet each, Lake Erie 565 feet, and Lake Ontario 232 feet. The three first named, having a depth of about 900 feet, have their beds about 300 feet below the surface of the ocean; Lake Ontario, with a depth of 500 feet, reaches a point 268 feet below the sea level. The depth of the St. Lawrence river bed, as related to the sea, is not ascertained. The most of the Swiss lakes, too, having a depth often of more than 1000 feet, come under the same category with the lakes under consideration above, waters from the mountains having gradually filled up chasms made at the time of the upheaval of the adjacent region. Some of these lake basins may be deep enough to lie below the level of the ocean.
Some bitter salt lakes on the Isthmus of Suez, forming a chain from the Red Sea to the southeast corner of the Mediterranean, long claimed attention from their supposed singularity. During the occupation of Egypt by the French in 1799, a survey of the district was made with the level, in view of a prospective canal across the isthmus, connecting the Nile with the Red Sea. An account of that survey was published by Le Père, in his greatDescription de l’Egypte. The result of the survey was very surprising; it assigned to the Gulf of Suez a height of 25 feet at ebb tide and 30½ feet at flood tide, above the level of the Mediterranean, a result which seemed to agree with Pliny’s account (vi. 23) of the elevation of the Red Sea above the level of lower Egypt. The salt swamps lying between the two seas, and known even to the ancients, lie, according to the same authority, 20 feet below the surface of the Mediterranean, and 50 feet below that of the Red Sea. These singular statements were not received without considerable doubt as to their correctness; but during the military disturbances in that region, no revision of the investigations could be made. Certain circumstances connected with an unusual inundation of the Nile in 1800, when its waters flowed as far as the transverse valley called the Wady Tumilet, in which the salt lakes lie, and where traces of the ancient canal, built by the Egyptians between the seas, could be seen, seemed to confirm the result of the survey of 1799. The inference was a natural one—that the sandy Isthmus of Suez was an accumulation of dunes, and of the deposits of inundations of both the Mediterranean and Red Seas, and that the salt morasses in the middle are but a trace of the primitive bottom. There were not wanting defenders of the old measurement, Favier being the most prominent. Since 1845 five surveys have been made, in reference to the projected canal. These all contradict the results of 1799, and show that there is but the difference of four-sevenths of a foot between the level of the two seas, and that there is the same agreement there as in all other parts of the earth. Many hypotheses, built on the old measurement, have accordingly fallen to the ground.
Between the two great and most sharply-marked physical features—the high plateaus and mountains and the lands of very little elevation—there are regions of transition very numerous and exceedingly varied.
The conception of highlands and of lowlands having a certain, constant, and absolute value, and it being immaterial whether the elevation be specially marked or not, provided it be uniform, the regions of transition find their most marked characteristics in their want of constancy, in their very change, and the rate at which the grade ascends from a low to a high elevation, or falls from a high to a low one. Their real value lies in the mutual compensation of highlands and lowlands, which is effected through the mediation of a third physical feature or system, which has received the name Lands of Gradation, or Terrace Lands, and which, by their gradual rise from the sea level, serve as the means of transition from the lowest lowlands to the loftiest plateaus and mountains.
Districts sloping to the sea, or lands of gradation, as we have called them, varying as they do in elevation and in relative situation to each other, are the true mediators between the districts but little above the level of the sea and others much more lofty. At the sources and the mouths of rivers they partake, more or less fully, in the characteristics of both highland and lowland. The manner of their mediation, as determined by the rate of the fall of water and by their direction, gives to every one of these regions of transition its peculiar character, determines itsconformation and its relation to the globe. And yet, no more than in lowlands and highlands, can we rid ourselves of some arbitrary data relating to the size of rivers, when we discriminate between those which we call large and those which we call small. As in all other geographical distinctions, we must here be content with arbitrary approximation, and with the ordinary usages of speech. The comparison of streams, in regard to their breadth and fullness, determines their volume; the comparison, in respect of length and tributary waters, determines the compass of the river system. The entire characteristics, breadth, depth of channel, length and extent of drainage, determine the status of the river, whether first, second, or third class, in relation first to those of the same continent, and then to those of the world. The Volga, for instance, is, in relation to Europe, a first-class river, but, like the Danube, in relation to the entire globe, is merely in the second or third rank. Not the length alone determines the importance of rivers. The Thames, one of the smallest streams in Europe, is one of the most important. And aside from commercial considerations, a river of insignificant size can have great influence in consequence of its relation to the entire adjacent region. The little Bavarian Isar, a river which, so far as the great world is concerned, seems to have no importance, receives on the left side the water of 860 tributary brooks, among which are 44 rivulets; on the right bank the water of 433: these 1293 brooks and rivulets pour themselves into the Isar through 103 direct tributaries, and not these alone, but the waters of 136 lakes are embraced within the Isar system! Yet the Isar is only one of 34 branches of the Danube, and of the fourth rank even among them, and the Danube is by no means one of the great rivers of the globe. A short butnavigable stream can have great influence over a territory limited in extent, and may make a long but shallow stream sink into insignificance in respect of comparative importance to the world. There are some great streams which are of first magnitude in all their characteristics—rivers which drain millions of square miles in their course to the sea. The number of such is small, however; there are scarcely fifty on the whole globe. Besides these, there is a large number of rivers much shorter, and of much less volume, but not deficient in the attributes which give a stream value to man, and which serve to mediate between highlands and lowlands, to fulfill the needs of navigation and to drain regions of more or less magnitude. These can be classed in four ranks: in the first place absolutely, and in the second place in relation to each continent. Yet, in classing them, it is necessary always to keep in mind that it is not size alone which gives a river its value, but a combination of all its characteristics, and its relative influence on the country through which it runs.
Looking at the direction of streams, we observe that there are some which flow northerly, as for instance those of Siberia, the Nile, the Rhine, the Elbe, and the Weser; there are those which flow southward—the Indus, Ganges, Euphrates, La Plata, Mississippi, and Volga, for example; there are those flowing eastward—Hoang-ho and Yang-tse-Kiang, the Amazon, the Orinoco, and the Danube, for instance; and some westward, instances of which may be found in the Gihon and Sihon, the Senegal, Gambia, Niger, the Colorado, the Seine, Loire, Garonne, and the Spanish rivers which enter the sea in Portugal.
And this characteristic, trite and unmeaning as it may at first seem, establishes, for the area which these rivers water, very diverse conditions. In like manner, too, theirposition, in relation to the oceans into which they flow, is very influential, in consequence of the action of the tide upon the lower course. The emergence of their head-waters at various altitudes, whether on plateaus of the first or second class, or on mountain tops covered with perpetual snow, gives rise to a great diversity of relations, that makes no one stream on the earth twin brother to any other. Rivers have an individuality which claims recognition, although they are usually summed up in one category.
This diversity in rivers becomes more apparent from a study of the diversified form of the terraces, or grades of transition, through which they pass on their way to the sea.
The great basin of the Nile is divided into three distinct parts or grades—Abyssinia, Nubia, and Egypt; and each of them has long been studied historically and physically. The great basin of the Rhine is also naturally divided into three grades—the Swiss highlands, the German moderate plateau, and the lowlands of Holland. In a similar manner there may almost always be traced in rivers three natural grades, and where they do not have, as in the cases just cited they do have, a historical significance, their physical influence is not hard to trace and to follow into all its analogies.
The word water-shed, now a familiar one, is applied to that point of division where contiguous springs pour their water in different directions. It is not even in a mountainous country necessarily coincident with the highest points of the chain, though it may be; the valleys may slope in such a way as to have more influence in determining the direction of running water than the mountains hard by. Every stream has its own water-shed system, and this system is the real boundary of its basin. If we trace thisbasin to its very limits on the highlands, we may find, not a mile away, the beginnings of another river, which shall flow in just the contrary direction, as for example in the case of the Rhine and Rhone, the Volga and the Dwina. The sources of the Missouri and of the Columbia lie close together, not a quarter of an hour’s walk apart; yet the waters of one flow into the Atlantic, of the other into the Pacific, and their mouths are almost 2500 miles apart. The Mongolians hold the water-shed in such estimation that they throw up a heap of stones wherever one occurs, establish it a shrine for prayer; and the Toongooses of Siberia never pass by one without casting a cedar branch upon the stone heap, that, to use their expression, “the holy mountain that parts the waters may not lessen, but increase.”
The main channel is the stream proper; the others are tributaries. The longest tributaries coming in from the region where the river proper rises, can be grouped intimately with the source of the main current, hardly distinguished from it in relative importance—the two, for instance, in the Nile, the five in the Indus, two in the Ganges, three in the Amazon, etc. All form in their confluence the real channel of the river. And the entire body of tributaries, taken in conjunction with the river proper, forms the river system, and the district which they all drain is their true reciprocal. The two, in their mutual action and reaction, form a whole, and are always thought of together. The source and the mouth are the beginning and the end of the whole system; the main channel and the circuit of water-shed, the center and the circumference of it. All the tributaries in their union constitute what may be called the arterial system of the river basin; the form of each and the characteristics of each are analogous to thoseof the whole, only in reduced pattern. The network which all the tributaries make is often surprisingly intricate. The symmetry with which the main characteristics of a river system are carried into the details, even of its smallest accessories, can only be compared to that observable in the architectural regularity of a tree, as it expands from the main trunk into the countless symmetrical branches.