Fig. 1.
Fig. 1.
A good illustration of the principle here alluded to may be sometimes seen in the neighbourhood of a volcano, when a section, whether natural or artificial, has laid open to view a succession of various-coloured layers of sand and ashes, which have fallen in showers upon uneven ground. Thus let A B (fig. 1.) be two ridges, with an intervening valley. These original inequalities of the surface have been gradually effaced by beds of sand and ashesc,d,e, the surface at e being quite level. It will be seen that although the materials of the first layers have accommodated themselves in a great degree to the shape of the ground A B, yet each bed is thickest at the bottom. At first a great many particles would be carried by their own gravity down the steep sides of A and B, and others would afterwards be blown by the wind as they fell off the ridges, and would settle in the hollow, which would thus become more and more effaced as the strata accumulated fromctoe. This levelling operation may perhaps be rendered more clear to the student by supposing a number of parallel trenches to be dug in a plain of moving sand, like the African desert, in which case the wind would soon cause all signs of these trenches to disappear, and the surface would be as uniform as before. Now, water inmotion can exert this levelling power on similar materials more easily than air, for almost all stones lose in water more than a third of the weight which they have in air, the specific gravity of rocks being in general as 21/2when compared to that of water, which is estimated at 1. But the buoyancy of sand or mud would be still greater in the sea, as the density of salt water exceeds that of fresh.
Yet, however uniform and horizontal may be the surface of new deposits in general, there are still many disturbing causes, such as eddies in the water, and currents moving first in one and then in another direction, which frequently cause irregularities. We may sometimes follow a bed of limestone, shale, or sandstone, for a distance of many hundred yards continuously; but we generally find at length that each individual stratum thins out, and allows the beds which were previously above and below it to meet. If the materials are coarse, as in grits and conglomerates, the same beds can rarely be traced many yards without varying in size, and often coming to an end abruptly. (Seefig. 2.)
Fig. 2.Section of strata of sandstone, grit, and conglomerate.
Fig. 2.
Section of strata of sandstone, grit, and conglomerate.
Fig. 3.Section of sand at Sandy Hill, near Biggleswade, Bedfordshire. Height 20feet. (Greensandformation.)
Fig. 3.
Section of sand at Sandy Hill, near Biggleswade, Bedfordshire. Height 20feet. (Greensandformation.)
Diagonal or Cross Stratification.—There is also another phenomenon of frequent occurrence. We find a series of larger strata, each of which is composed of a number of minor layers placed obliquely to the general planes of stratification. To this diagonal arrangement the name of "false or cross stratification" has been given. Thus in the annexed section (fig. 3.) we see seven or eightlarge beds of loose sand, yellow and brown, and the linesa,b,c, mark some of the principal planes of stratification, which are nearly horizontal. But the greater part of the subordinate laminæ do not conform to these planes, but have often a steep slope, the inclination being sometimes towards opposite points of the compass. When the sand is loose and incoherent, as in the case here represented, the deviation from parallelism of the slanting laminæ cannot possibly be accounted for by any re-arrangement of the particles acquired during the consolidation of the rock. In what manner then can such irregularities be due to original deposition? We must suppose that at the bottom of the sea, as well as in the beds of rivers, the motions of waves, currents, and eddies often cause mud, sand, and gravel to be thrown down in heaps on particular spots, instead of being spread out uniformly over a wide area. Sometimes, when banks are thus formed, currents may cut passages through them, just as a river forms its bed. Suppose the bank A (fig. 4.) to be thus formed with a steep sloping side, and the water being in a tranquil state, the layer of sediment No. 1. is thrown down upon it, conforming nearly to its surface. Afterwards the other layers, 2, 3, 4, may be deposited in succession, so that the bank B C D is formed. If the current then increases in velocity, it may cut away the upper portion of this mass down to the dotted linee(fig. 4.), and deposit the materials thus removed farther on, so as to form the layers 5, 6, 7, 8. We have now the bank B C D E (fig. 5.), of which the surface is almost level, and on which the nearly horizontal layers, 9, 10, 11, may then accumulate. It was shown infig. 3.that the diagonal layers of successive strata may sometimes have an opposite slope. This is well seen in some cliffs of loose sand on the Suffolk coast. A portion of one of these is represented infig. 6., where the layers, of which there are about six in the thickness of an inch, are composed of quartzose grains. This arrangement may have been due to the altered direction of the tides and currents in the same place.
Fig. 4.
Fig. 4.
Fig. 5.
Fig. 5.
Fig. 6.Cliff between Mismer and Dunwich.
Fig. 6.
Cliff between Mismer and Dunwich.
Fig. 7.Section from Monte Calvo to the sea by the valley of Magnan, near Nice.A. Dolomite and sandstone. (Greensand formation?)a,b,d. Beds of gravel and sand.c.Fine marl and sand of St. Madeleine, with marine shells.
Fig. 7.
Section from Monte Calvo to the sea by the valley of Magnan, near Nice.
The description above given of the slanting position of the minor layers constituting a single stratum is in certain cases applicable on a much grander scale to masses several hundred feet thick, and many miles in extent. A fine example may be seen at the base of the Maritime Alps near Nice. The mountains here terminate abruptly in the sea, so that a depth of many hundred fathoms is often found within a stone's throw of the beach, and sometimes a depth of 3000 feet within half a mile. But at certain points, strata of sand, marl, or conglomerate, intervene between the shore and the mountains, as in the annexedfig. 7., where a vast succession of slanting beds of gravel and sand may be traced from the sea to Monte Calvo, a distance of no less than 9 miles in a straight line. The dip of these beds is remarkably uniform, being always southward or towards the Mediterranean, at an angle of about 25°. They are exposed to view in nearly vertical precipices, varying from 200 to 600 feet in height, which bound the valley through which the river Magnan flows. Although in a general view, the strata appear to be parallel and uniform, they are nevertheless found, when examined closely, to be wedge-shaped, and to thin out when followed for a few hundred feet or yards, so that we may suppose them to have been thrown down originally upon the side of a steep bank, where a river or alpine torrent discharged itself into a deep and tranquil sea, and formed a delta, which advanced gradually from the base of Monte Calvo to a distance of 9 miles from the original shore. If subsequently this part of the Alps and bed of the sea were raised 700 feet, the coast would acquire its present configuration, the delta would emerge, and a deep channel might then be cut through it by a river.
It is well known that the torrents and streams, which now descend from the alpine declivities to the shore, bring down annually, when the snow melts, vast quantities of shingle and sand, and then, as they subside, fine mud, while in summer they are nearly or entirely dry; so that it may be safely assumed, that deposits like those of the valley of the Magnan, consisting of coarse gravel alternating with fine sediment, are still in progress at many points, as, for instance, at the mouth of the Var. They must advance upon the Mediterranean in the form of great shoals terminating in a steep talus; such being theoriginal mode of accumulation of all coarse materials conveyed into deep water, especially where they are composed in great part of pebbles, which cannot be transported to indefinite distances by currents of moderate velocity. By inattention to facts and inferences of this kind, a very exaggerated estimate has sometimes been made of the supposed depth of the ancient ocean. There can be no doubt, for example, that the strataa,fig. 7., or those nearest to Monte Calvo, are older than those indicated byb, and these again were formed beforec; but the vertical depth of gravel and sand in any one place cannot be proved to amount even to 1000 feet, although it may perhaps be much greater, yet probably never exceeding at any point 3000 or 4000 feet. But were we to assume that all the strata were once horizontal, and that their present dip or inclination was due to subsequent movements, we should then be forced to conclude, that a sea 9 miles deep had been filled up with alternate layers of mud and pebbles thrown down one upon another.
In the locality now under consideration, situated a few miles to the west of Nice, there are many geological data, the details of which cannot be given in this place, all leading to the opinion, that when the deposit of the Magnan was formed, the shape and outline of the alpine declivities and the shore greatly resembled what we now behold at many points in the neighbourhood. That the beds, a, b, c, d, are of comparatively modern date is proved by this fact, that in seams of loamy marl intervening between the pebbly beds are fossil shells, half of which belong to species now living in the Mediterranean.
Fig. 8.Slab of ripple-marked (new red) sandstone from Cheshire.
Fig. 8.
Slab of ripple-marked (new red) sandstone from Cheshire.
Ripple mark.—The ripple mark, so common on the surface of sandstones of all ages (seefig. 8.), and which is so often seen on thesea-shore at low tide, seems to originate in the drifting of materials along the bottom of the water, in a manner very similar to that which may explain the inclined layers above described. This ripple is not entirely confined to the beach between high and low water mark, but is also produced on sands which are constantly covered by water. Similar undulating ridges and furrows may also be sometimes seen on the surface of drift snow and blown sand. The following is the manner in which I once observed the motion of the air to produce this effect on a large extent of level beach, exposed at low tide near Calais. Clouds of fine white sand were blown from the neighbouring dunes, so as to cover the shore, and whiten a dark level surface of sandy mud, and this fresh covering of sand was beautifully rippled. On levelling all the small ridges and furrows of this ripple over an area of several yards square, I saw them perfectly restored in about ten minutes, the general direction of the ridges being always at right angles to that of the wind. The restoration began by the appearance here and there of small detached heaps of sand, which soon lengthened and joined together, so as to form long sinuous ridges with intervening furrows. Each ridge had one side slightly inclined, and the other steep; the lee-side being always steep, asb, c,—d, e; the windward-side a gentle slope, asa, b,—c, d,fig. 9.When a gust of wind blew with sufficient force to drive along a cloud of sand, all the ridges were seen to be in motion at once, each encroaching on the furrow before it, and, in the course of a few minutes, filling the place which the furrows had occupied. The mode of advance was by the continual drifting of grains of sand up the slopesa bandc d, many of which grains, when they arrived atbandd, fell over the scarpsb candd e, and were under shelter from the wind; so that they remained stationary, resting, according to their shape and momentum, on different parts of the descent, and a few only rolling to the bottom. In this manner each ridge was distinctly seen to move slowly on as often as the force of the wind augmented. Occasionally part of a ridge, advancing more rapidly than the rest, overtook the ridge immediately before it, and became confounded with it, thus causing those bifurcations and branches which are so common, and two of which are seen in the slab,fig. 8.We may observe this configuration in sandstones of all ages, and in them also, as now on the sea-coast, we may often detect two systems of ripples interfering with each other; one more ancient and half effaced, and a newer one, in which the grooves and ridges are more distinct, and in a different direction. This crossing of two sets of ripples arises from a change of wind, and the new direction in which the waves are thrown on the shore.
Fig. 9.
Fig. 9.
The ripple mark is usually an indication of a sea-beach, or of water from 6 to 10 feet deep, for the agitation caused by waves evenduring storms extends to a very slight depth. To this rule, however, there are some exceptions, and recent ripple marks have been observed at the depth of 60 or 70 feet. It has also been ascertained that currents or large bodies of water in motion may disturb mud and sand at the depth of 300 or even 450 feet.[21-A]
Successive deposition indicated by fossils — Limestones formed of corals and shells Proofs of gradual increase of strata derived from fossils — Serpula attached to spatangus — Wood bored by Teredina — Tripoli and semi-opal formed of infusoria — Chalk derived principally from organic bodies — Distinction of freshwater from marine formations — Genera of freshwater and land shells — Rules for recognizing marine testacea — Gyrogonite and chara — Freshwater fishes — Alternation of marine and freshwater deposits — Lym-Fiord.
Successive deposition indicated by fossils — Limestones formed of corals and shells Proofs of gradual increase of strata derived from fossils — Serpula attached to spatangus — Wood bored by Teredina — Tripoli and semi-opal formed of infusoria — Chalk derived principally from organic bodies — Distinction of freshwater from marine formations — Genera of freshwater and land shells — Rules for recognizing marine testacea — Gyrogonite and chara — Freshwater fishes — Alternation of marine and freshwater deposits — Lym-Fiord.
Havingin the last chapter considered the forms of stratification so far as they are determined by the arrangement of inorganic matter, we may now turn our attention to the manner in which organic remains are distributed through stratified deposits. We should often be unable to detect any signs of stratification or of successive deposition, if particular kinds of fossils did not occur here and there at certain depths in the mass. At one level, for example, univalve shells of some one or more species predominate; at another, bivalve shells; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating strata.
It may appear inconceivable to a beginner how mountains, several thousand feet thick, can have become filled with fossils from top to bottom; but the difficulty is removed, when he reflects on the origin of stratification, as explained in the last chapter, and allows sufficient time for the accumulation of sediment. He must never lose sight of the fact that, during the process of deposition, each separate layer was once the uppermost, and covered immediately by the water in which aquatic animals lived. Each stratum in fact, however far it may now lie beneath the surface, was once in the state of shingle, or loose sand or soft mud at the bottom of the sea, in which shells and other bodies easily became enveloped.
By attending to the nature of these remains, we are often enabled to determine whether the deposition was slow or rapid, whether it took place in a deep or shallow sea, near the shore or far from land, and whether the water was salt, brackish, or fresh. Some limestonesconsist almost exclusively of corals, and in many cases it is evident that the present position of each fossil zoophyte has been determined by the manner in which it grew originally. The axis of the coral, for example, if its natural growth is erect, still remains at right angles to the plane of stratification. If the stratum be now horizontal, the round spherical heads of certain species continue uppermost, and their points of attachment are directed downwards. This arrangement is sometimes repeated throughout a great succession of strata. From what we know of the growth of similar zoophytes in modern reefs, we infer that the rate of increase was extremely slow, and some of the fossils must have flourished for ages like forest trees, before they attained so large a size. During these ages, the water remained clear and transparent, for such corals cannot live in turbid water.
Fig. 10.FossilGryphæa, covered both on the outside and inside with fossil serpulæ.
Fig. 10.
FossilGryphæa, covered both on the outside and inside with fossil serpulæ.
In like manner, when we see thousands of full-grown shells dispersed every where throughout a long series of strata, we cannot doubt that time was required for the multiplication of successive generations; and the evidence of slow accumulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of the ocean after death before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with serpulæ, or barnacles (acorn-shells), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear water, when the testacea, now adhering to it, grew from an embryo state to full maturity. Attached shells which are merely external, like some of the serpulæ (a) in the annexed figure (fig. 10.), may often have grown upon an oyster or other shell while the animal within was still living; but if they are found on the inside, it could only happen after the death of the inhabitant of the shell which affords the support. Thus, infig. 10., it will be seen that two serpulæ have grown on the interior, one of them exactly on the place where the adductor muscle of theGryphæa(a kind of oyster) was fixed.
Some fossil shells, even if simply attached to theoutsideof others, bear full testimony to the conclusion above alluded to, namely, that an interval elapsed between the death of the creature to whose shell they adhere, and the burial of the same in mud or sand. The sea-urchins orEchini, so abundant in white chalk, afford a good illustration.It is well known that these animals, when living, are invariably covered with numerous spines, which serve as organs of motion, and are supported by rows of tubercles, which last are only seen after the death of the sea-urchin, when the spines have dropped off. Infig. 12.a living species ofSpatangus, common on our coast, is represented with one half of its shell stripped of the spines. Infig. 11.a fossil of the same genus from the white chalk of England shows the naked surface which the individuals of this family exhibit when denuded of their bristles. The full-grownSerpula, therefore, which now adheres externally, could not have begun to grow till theSpatangushad died, and the spines were detached.
Fig. 11.Serpulaattached to a fossilSpatangusfrom the chalk.
Fig. 11.
Serpulaattached to a fossilSpatangusfrom the chalk.
Fig. 12.RecentSpatanguswith the spines removed from one side.b.Spine and tubercles, nat. size.a.The same magnified.
Fig. 12.
RecentSpatanguswith the spines removed from one side.
Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea-urchin in the chalk (seefig. 13.), which has fixed to it the lower valve of aCrania, a genus of bivalve mollusca. The upper valve (b,fig. 13.) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. Then the youngCraniaadhered to the bared shell, grew and perished in its turn; after which the upper valve was separated from the lower before theEchinusbecame enveloped in chalky mud.
Fig. 13.a.Echinusfrom the chalk, with lower valve of theCraniaattached.b.Upper valve of theCraniadetached.
Fig. 13.
It may be well to mention one more illustration of the manner in which single fossils may sometimes throw light on a former state of things, both in the bed of the ocean and on some adjoining land. We meet with many fragments of wood bored by ship-worms at various depths in the clay on which London is built. Entire branches and stems of trees, several feet in length, are sometimes dug out, drilled all over by the holes of these borers, the tubes and shells of the mollusk still remaining in the cylindrical hollows. Infig. 15.e, a representation is given of a piece of recent wood pierced by theTeredo navalis, or common ship-worm, which destroys wooden piles and ships. When the cylindrical tubedhas been extracted from the wood, a shell is seen at the larger extremity, composed of two pieces, as shown atc. In like manner, a piece of fossil wood (a,fig. 14.)has been perforated by an animal of a kindred but extinct genus, calledTeredinaby Lamarck. The calcareous tube of this mollusk was united and as it were soldered on to the valves of the shell (b), which therefore cannot be detached from the tube, like the valves of the recentTeredo. The wood in this fossil specimen is now converted into a stony mass, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when theTeredinælived upon it, perforating it in all directions. Again, before the infant colony settled upon the drift wood, the branch of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: and thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate.
Fossil and recent wood drilled by perforating Mollusca.Fig. 14.a. Fossil wood from London clay, bored byTeredina.b. Shell and tube ofTeredina personata, the right-hand figure the ventral, the left the dorsal view.Fig. 15.e. Recent wood bored byTeredo.d. Shell and tube ofTeredo navalis, from the same.c. Anterior and posterior view of the valves of same detached from the tube.
Fossil and recent wood drilled by perforating Mollusca.
It has been already remarked that there are rocks in the interior of continents, at various depths in the earth, and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such masses may be compared to modern oyster-beds and coral reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stony deposits in the earth's crust, now proved to have been derived from plants and animals, of which the organic origin was not suspected until of late years, even by naturalists. Great surprise was therefore created by the recent discovery of Professor Ehrenberg of Berlin, that a certain kind of siliceous stone, called tripoli, was entirely composed of millions of the remains of organic beings, which the Prussian naturalist refers to microscopic Infusoria, but which most others now believe to be plants. They abound in freshwater lakes and ponds in England and other countries, and are termed Diatomaceæ by those naturalists who believe in their vegetable origin. The substance alluded to haslong been well known in the arts, being used in the form of powder for polishing stones and metals. It has been procured, among other places, from Bilin, in Bohemia, where a single stratum, extending over a wide area, is no less than 14 feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous plates or frustules of the above-mentioned Diatomaceæ, united together without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of theGaillonella distans(seefig. 17.) in every cubic inch, which weighs about 220 grains, or about 187 millions in a single grain. At every stroke, therefore, that we make with this polishing powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.
Fig. 16.Bacillaria vulgaris?Fig. 17.Gaillonella distans.Fig. 18.Gaillonella ferruginea.These figures are magnified nearly 300 times, except the lower figure ofG. ferruginea(fig. 18.a), which is magnified 2000 times.
Fig. 16.Bacillaria vulgaris?
Fig. 17.Gaillonella distans.
Fig. 18.Gaillonella ferruginea.
These figures are magnified nearly 300 times, except the lower figure ofG. ferruginea(fig. 18.a), which is magnified 2000 times.
Fragment of semi-opal from the great bed of Tripoli, Bilin.Fig. 19. Natural size.Fig. 20. The same magnified, showing circular articulations of a species ofGaillonella, and spiculæ ofSpongilla.
Fragment of semi-opal from the great bed of Tripoli, Bilin.
Fig. 19. Natural size.
Fig. 20. The same magnified, showing circular articulations of a species ofGaillonella, and spiculæ ofSpongilla.
The remains of these Diatomaceæ are of pure silex, and their forms are various, but very marked and constant in particular genera and species. Thus, in the familyBacillaria(seefig. 16.), the fossils preserved in tripoli are seen to exhibit the same divisions and transverse lines which characterize the living species of kindred form. With these, also, the siliceous spiculæ or internal supports of the freshwater sponge, orSpongillaof Lamarck, are sometimes intermingled (see the needle-shaped bodies infig. 20.). These flinty cases and spiculæ, although hard, are very fragile, breaking like glass, and are therefore admirably adapted, when rubbed, for wearing down into a fine powder fit for polishing the surface of metals.
Besides the tripoli, formed exclusively of the fossilsabove described, there occurs in the upper part of the great stratum at Bilin another heavier and more compact stone, a kind of semi-opal, in which innumerable parts of Diatomaceæ and spiculæ of theSpongillaare filled with, and cemented together by, siliceous matter. It is supposed that the siliceous remains of the most delicate Diatomaceæ have been dissolved by water, and have thus given rise to this opal in which the more durable fossils are preserved like insects in amber. This opinion is confirmed by the fact that the organic bodies decrease in number and sharpness of outline in proportion as the opaline cement increases in quantity.
In the Bohemian tripoli above described, as in that of Planitz in Saxony, the species of Diatomaceæ (or Infusoria, as termed by Ehrenberg) are freshwater; but in other countries, as in the tripoli of the Isle of France, they are of marine species, and they all belong to formations of thetertiaryperiod, which will be spoken of hereafter.
A well-known substance, called bog-iron ore, often met with in peat-mosses, has also been shown by Ehrenberg to consist of innumerable articulated threads, of a yellow ochre colour, composed partly of flint and partly of oxide of iron. These threads are the cases of a minute microscopic body, calledGaillonella ferruginea(fig. 18.).
CytheridæandForaminiferafrom the chalk.Fig. 21.Cythere, Müll.Cytherina, Lam.Fig. 22. Portion ofNodosaria.Fig. 23.Cristellaria rotulata.Fig. 24.Rosalina.
CytheridæandForaminiferafrom the chalk.
Fig. 21.Cythere, Müll.Cytherina, Lam.
Fig. 22. Portion ofNodosaria.
Fig. 23.Cristellaria rotulata.
Fig. 24.Rosalina.
It is clear that much time must have been required for the accumulation of strata to which countless generations of Diatomaceæ have contributed their remains; and these discoveries lead us naturally to suspect that other deposits, of which the materials have usually been supposed to be inorganic, may in reality have been derived from microscopic organic bodies. That this is the case with the white chalk, has often been imagined, this rock having been observed to abound in a variety of marine fossils, such as shells, echini, corals, sponges, crustacea, and fishes. Mr. Lonsdale, on examining, in Oct. 1835, in the museum of the Geological Society of London, portions of white chalk from different parts of England, found, on carefully pulverizing them in water, that what appear to the eye simply as white grains were, in fact, well preserved fossils. He obtained above a thousand of these from each pound weight of chalk, some being fragments of minute corallines, others entire Foraminifera and Cytheridæ. The annexed drawings will give an idea of the beautiful forms of many of these bodies. The figuresaarepresent their natural size, but, minute as they seem, the smallest of them, such asa,fig. 24., are gigantic in comparison with the cases of Diatomaceæ before mentioned. It has, moreover, been lately discovered that thechambers into which these Foraminifera are divided are actually often filled with thousands of well-preserved organic bodies, which abound in every minute grain of chalk, and are especially apparent in the white coating of flints, often accompanied by innumerable needle-shaped spiculæ of sponges. After reflecting on these discoveries, we are naturally led on to conjecture that, as the formless cement in the semi-opal of Bilin has been derived from the decomposition of animal and vegetable remains, so also even those parts of chalk flints in which no organic structure can be recognized may nevertheless have constituted a part of microscopic animalcules.
"The dust we tread upon was once alive!"—Byron.
"The dust we tread upon was once alive!"—Byron.
How faint an idea does this exclamation of the poet convey of the real wonders of nature! for here we discover proofs that the calcareous and siliceous dust of which hills are composed has not only been once alive, but almost every particle, albeit invisible to the naked eye, still retains the organic structure which, at periods of time incalculably remote, was impressed upon it by the powers of life.
Freshwater and marine fossils.—Strata, whether deposited in salt or fresh water, have the same forms; but the imbedded fossils are very different in the two cases, because the aquatic animals which frequent lakes and rivers are distinct from those inhabiting the sea. In the northern part of the Isle of Wight a formation of marl and limestone, more than 50 feet thick, occurs, in which the shells are principally, if not all, of extinct species. Yet we recognize their freshwater origin, because they are of the same genera as those now abounding in ponds and lakes, either in our own country or in warmer latitudes.
In many parts of France, as in Auvergne, for example, strata of limestone, marl, and sandstone are found, hundreds of feet thick, which contain exclusively freshwater and land shells, together with the remains of terrestrial quadrupeds. The number of land shells scattered through some of these freshwater deposits is exceedingly great; and there are districts in Germany where the rocks scarcely contain any other fossils except snail-shells (helices); as, for instance, the limestone on the left bank of the Rhine, between Mayence and Worms, at Oppenheim, Findheim, Budenheim, and other places. In order to account for this phenomenon, the geologist has only to examine the small deltas of torrents which enter the Swiss lakes when the waters are low, such as the newly-formed plain where the Kander enters the Lake of Thun. He there sees sand and mud strewed over with innumerable dead land shells, which have been brought down from valleys in the Alps in the preceding spring, during the melting of the snows. Again, if we search the sands on the borders of the Rhine, in the lower part of its course, we find countless land shells mixed with others of species belonging to lakes, stagnant pools, and marshes. These individuals have been washedaway from the alluvial plains of the great river and its tributaries, some from mountainous regions, others from the low country.
Although freshwater formations are often of great thickness, yet they are usually very limited in area when compared to marine deposits, just as lakes and estuaries are of small dimensions in comparison with seas.
We may distinguish a freshwater formation, first, by the absence of many fossils almost invariably met with in marine strata. For example, there are no sea-urchins, no corals, and scarcely any zoophytes; no chambered shells, such as the nautilus, nor microscopic Foraminifera. But it is chiefly by attending to the forms of the mollusca that we are guided in determining the point in question. In a freshwater deposit, the number of individual shells is often as great, if not greater, than in a marine stratum; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of recent freshwater and land shells are few when contrasted with the marine. Thus, the genera of true mollusca according to Blainville's system, excluding those of extinct species and those without shells, amount to about 200 in number, of which the terrestrial and freshwater genera scarcely form more than a sixth.[28-A]
Fig. 25.Cyclas obovata;fossil. Hants.
Fig. 25.
Cyclas obovata;fossil. Hants.
Fig. 26.Cyrena consobrina;fossil. Grays,Essex.
Fig. 26.
Cyrena consobrina;fossil. Grays,Essex.
Fig. 27.Anodonta Cordierii;fossil. Paris.
Fig. 27.
Anodonta Cordierii;fossil. Paris.
Fig. 28.Anodonta latimarginatus;recent. Bahia.
Fig. 28.
Anodonta latimarginatus;recent. Bahia.
Fig. 29.Unio littoralis;recent. Auvergne.
Fig. 29.
Unio littoralis;recent. Auvergne.
Almost all bivalve shells, or those of acephalous mollusca, are marine, about ten only out of ninety genera being freshwater. Among these last, the four most common forms, both recent and fossil, areCyclas,Cyrena,Unio, andAnodonta(see figures); the two first and two last of which are so nearly allied as to pass into each other.
Fig. 30.Gryphæa incurva, Sow. (G. arcuata, Lam.) upper valve. Lias.
Fig. 30.
Gryphæa incurva, Sow. (G. arcuata, Lam.) upper valve. Lias.
Lamarck divided the bivalve mollusca into theDimyary, or those having two large muscular impressions in each valve, asa bin the Cyclas,fig. 25., and theMonomyary, such as the oyster and scallop, in which there is only one of these impressions, as is seen infig. 30.Now, as none of these last, or the unimuscular bivalves, are freshwater, we may at once presume a deposit in which we find any of them to be marine.
Fig. 31.Planorbis euomphalus;fossil. Isleof Wight.
Fig. 31.
Planorbis euomphalus;fossil. Isleof Wight.
Fig. 32.Lymnea longiscata;fossil. Hants.
Fig. 32.
Lymnea longiscata;fossil. Hants.
Fig. 33.Paludina lenta;fossil. Hants.
Fig. 33.
Paludina lenta;fossil. Hants.
The univalve shells most characteristic of freshwater deposits are,Planorbis,Lymnea, andPaludina. (See figures.) But to these are occasionally addedPhysa,Succinea,Ancylus,Valvata,Melanopsis,Melania, andNeritina. (See figures.)
Fig. 34.Succinea amphibia;fossil. Loess,Rhine.
Fig. 34.
Succinea amphibia;fossil. Loess,Rhine.
Fig. 35.Ancylus elegans;fossil. Hants.
Fig. 35.
Ancylus elegans;fossil. Hants.
Fig. 36.Valvata; fossil. Grays, Essex.
Fig. 36.
Valvata; fossil. Grays, Essex.
Fig. 37.Physa hypnorum; recent.
Fig. 37.
Physa hypnorum; recent.
Fig. 38.Auricula;recent. Ava.
Fig. 38.
Auricula;recent. Ava.
Fig. 39.Melania inquinata. ParisBasin.
Fig. 39.
Melania inquinata. ParisBasin.
Fig. 40.Physa columnaris. ParisBasin.
Fig. 40.
Physa columnaris. ParisBasin.
Fig. 41.Melanopsis buccinoidea;recent. Asia.
Fig. 41.
Melanopsis buccinoidea;recent. Asia.
In regard to one of these, theAncylus(fig. 35.), Mr. Gray observes that it sometimes differs in no respect from the marineSiphonaria, except in the animal. The shell, however, of theAncylusis usually thinner.[29-A]