Fig. 472.a.Secondary conglomerate.b.Thin seams of volcanic sand and scoriæ.
Fig. 472.
Although the volcanos of Catalonia have broken out through sandstone, shale, and limestone, as have those of the Eifel, in Germany, to be described in the sequel, there is a remarkable difference in the nature of the ejections composing the cones in these two regions. In the Eifel, the quantity of pieces of sandstone and shale thrown out from the vents is often so immense as far to exceed in volume the scoriæ, pumice, and lava; but I sought in vain in the cones near Olot for a single fragment of any extraneous rock; and Don Francisco Bolos, an eminent botanist of Olot, informed me that he had never been able to detect any. Volcanic sand and ashes are not confined to the cones, but have been sometimes scattered by the wind over the country, and drifted into narrow valleys, as is seen between Olot and Cellent, where the annexed section (fig. 472.) is exposed. The light cindery volcanic matter rests in thin regular layers, just as it alighted on the slope formed by the solid conglomerate. No flood could have passed throughthe valley since the scoriæ fell, or these would have been for the most part removed.
Fig. 473.Section above the bridge of Cellent.a.Scoriaceous lava.b.Schistose basalt.c.Columnar basalt.d.Scoria, vegetable soil, and alluvium.e.Nummulitic limestone.f.Micaceous grey sandstone.
Fig. 473.
Section above the bridge of Cellent.
The currents of lava in Catalonia, like those of Auvergne, the Vivarais, Iceland, and all mountainous countries, are of considerable depth in narrow defiles, but spread out into comparatively thin sheets in places where the valleys widen. If a river has flowed on nearly level ground, as in the great plain near Olot, the water has only excavated a channel of slight depth; but where the declivity is great, the stream has cut a deep section, sometimes by penetrating directly through the central part of a lava-current, but more frequently by passing between the lava and the secondary rock which bounds the valley. Thus, in the accompanying section, at the bridge of Cellent, six miles east of Olot, we see the lava on one side of the small stream; while the inclined stratified rocks constitute the channel and opposite bank. The upper part of the lava at that place, as is usual in the currents of Etna and Vesuvius, is scoriaceous; farther down it becomes less porous, and assumes a spheroidal structure; still lower it divides in horizontal plates, each about 2 inches in thickness, and is more compact. Lastly, at the bottom is a mass of prismatic basalt about 5 feet thick. The vertical columns often rest immediately on the subjacent secondary rocks; but there is sometimes an intervention of such sand and scoriæ as cover the country during volcanic eruptions, and which when unprotected, as here, by superincumbent lava, is washed away from the surface of the land. Sometimes, the beddcontains a few pebbles and angular fragments of rock; in other places fine earth, which may have constituted an ancient vegetable soil.
In several localities, beds of sand and ashes are interposed between the lava and subjacent stratified rock, as may be seen if we follow the course of the lava-current which descends from Las Planas towards Amer, and stops two miles short of that town. The river there has often cut through the lava, and through 18 feet of underlyinglimestone. Occasionally an alluvium, several feet thick, is interspersed between the igneous and marine formation; and it is interesting to remark that in this, as in other beds of pebbles occupying a similar position, there are no rounded fragments of lava; whereas in the most modern gravel-beds of rivers of this country, volcanic pebbles are abundant.
The deepest excavation made by a river through lava, which I observed in this part of Spain, is that seen in the bottom of a valley near San Feliu de Palleróls, opposite the Castell de Stolles. The lava there has filled up the bottom of a valley, and a narrow ravine has been cut through it to the depth of 100 feet. In the lower part the lava has a columnar structure. A great number of ages were probably required for the erosion of so deep a ravine; but we have no reason to infer that this current is of higher antiquity than those of the plain near Olot. The fall of the ground, and consequent velocity of the stream, being in this case greater, a more considerable volume of rock may have been removed in the same time.
Fig. 474.Section at Castell Follit.A. Church and town of Castell Follit, overlooking precipices of basalt.B. Small island, on each side of which branches of the river Teronel flow to meet the Fluvia.c.Precipice of basaltic lava, chiefly columnar, about 130 feet in height.d.Ancient alluvium, underlying the lava-current.e.Inclined strata of secondary sandstone.
Fig. 474.
Section at Castell Follit.
I shall describe one more section to elucidate the phenomena of this district. A lava-stream, flowing from a ridge of hills on the east of Olot, descends a considerable slope, until it reaches the valley of the river Fluvia. Here, for the first time, it comes in contact with running water, which has removed a portion, and laid open its internal structure in a precipice about 130 feet in height, at the edge of which stands the town of Castell Follit.
By the junction of the rivers Fluvia and Teronel, the mass of lava has been cut away on two sides; and the insular rockB(fig. 474.) has been left, which was probably never so high as the cliffA, as it may have constituted the lower part of the sloping side of the original current.
From an examination of the vertical cliffs, it appears that the upper part of the lava on which the town is built is scoriaceous,passing downwards into a spheroidal basalt; some of the huge spheroids being no less than 6 feet in diameter. Below this is a more compact basalt, with crystals of olivine. There are in all five distinct ranges of basalt, the uppermost spheroidal, and the rest prismatic, separated by thinner beds not columnar, and some of which are schistose. These were probably formed by successive flows of lava, whether during the same eruption or at different periods. The whole mass rests on alluvium, ten or twelve feet in thickness, composed of pebbles of limestone and quartz, but without any intermixture of igneous rocks; in which circumstance alone it appears to differ from the modern gravel of the Fluvia.
Bufadors.—The volcanic rocks near Olot have often a cavernous structure, like some of the lavas of Etna; and in many parts of the hill of Batet, in the environs of the town, the sound returned by the earth, when struck, is like that of an archway. At the base of the same hill are the mouths of several subterranean caverns, about twelve in number, which are called in the country "bufadors," from which a current of cold air issues during summer, but which in winter is said to be scarcely perceptible. I visited one of these bufadors in the beginning of August, 1830, when the heat of the season was unusually intense, and found a cold wind blowing from it, which may easily be explained; for as the external air, when rarefied by heat, ascends, the pressure of the colder and heavier air of the caverns in the interior of the mountain causes it to rush out to supply its place.
In regard to the age of these Spanish volcanos, attempts have been made to prove, that in this country, as well as in Auvergne and the Eifel, the earliest inhabitants were eye-witnesses to the volcanic action. In the year 1421, it is said, when Olot was destroyed by an earthquake, an eruption broke out near Amer, and consumed the town. The researches of Don Francisco Bolos have, I think, shown, in the most satisfactory manner, that there is no good historical foundation for the latter part of this story; and any geologist who has visited Amer must be convinced that there never was any eruption on that spot. It is true that, in the year above mentioned, the whole of Olot, with the exception of a single house, was cast down by an earthquake; one of those shocks which, at distant intervals during the last five centuries, have shaken the Pyrenees, and particularly the country between Perpignan and Olot, where the movements, at the period alluded to, were most violent.
The annihilation of the town may, perhaps, have been due to the cavernous nature of the subjacent rocks; for Catalonia is beyond the line of those European earthquakes which have, within the period of history, destroyed towns throughout extensive areas.
As we have no historical records, then, to guide us in regard to the extinct volcanos, we must appeal to geological monuments. The annexed diagram will present to the reader, in a synoptical form, the results obtained from numerous sections.
The more modern alluvium (d) is partial, and has been formed bythe action of rivers and floods upon the lava; whereas the older gravel (b) was strewed over the country before the volcanic eruptions. In neither have any organic remains been discovered; so that we can merely affirm, as yet, that the volcanos broke out after the elevation of some of the newest rocks of the nummulitic (Eocene?) series of Catalonia, and before the formation of an alluvium (d) of unknown date. The integrity of the cones merely shows that the country has not been agitated by violent earthquakes, or subjected to the action of any great transient flood since their origin.
Fig. 475.Superposition of rocks in the volcanic district of Catalonia.a.Sandstone and nummulitic limestone.b.Older alluvium without volcanic pebbles.c.Cones of scoriæ and lava.d.Newer alluvium.
Fig. 475.
Superposition of rocks in the volcanic district of Catalonia.
East of Olot, on the Catalonian coast, marine tertiary strata occur, which, near Barcelona, attain the height of about 500 feet. From the shells which I collected, these strata appear to correspond in age with the Subapennine beds; and it is not improbable that their upheaval from beneath the sea took place during the period of volcanic eruption round Olot. In that case these eruptions may have occurred at the close of the Older Pliocene era, but perhaps subsequently, for their age is at present quite uncertain.
Miocene period—Volcanic rocks of the Eifel.—The chronological relations of the volcanic rocks of the Lower Rhine and the Eifel are also involved in a considerable degree of ambiguity; but we know that some portion of them were coeval with the deposition of a tertiary formation, called "Brown-Coal" by the Germans, which probably belongs to the Miocene, if not referable to the Upper Eocene, epoch.
This Brown-Coal is seen on both sides of the Rhine, in the neighbourhood of Bonn, resting unconformably on highly inclined and vertical strata of Silurian and Devonian rocks. Its position, and the space occupied by the volcanic rocks, both of the Westerwald and Eifel, will be seen by referring to the map in the next page (fig. 476.), for which I am indebted to Mr. Horner, whose residence in the country has enabled him to verify the maps of MM. Noeggerath and Von Oeynhausen, from which that now given has been principally compiled.
The Brown-Coal formation consists of beds of loose sand, sandstone, and conglomerate, clay with nodules of clay-ironstone, and occasionally silex. Layers of light brown, and sometimes black lignite, are interstratified with the clays and sands, and often irregularly diffused through them. They contain numerous impressions ofleaves and stems of trees, and are extensively worked for fuel, whence the name of the formation.
Fig. 476. Map of the volcanic region of the Upper and Lower Eifel.N.B.The country in that part of the map which is left blank is composed of inclined Silurian and Devonian rocks.
Fig. 476. Map of the volcanic region of the Upper and Lower Eifel.
N.B.The country in that part of the map which is left blank is composed of inclined Silurian and Devonian rocks.
In several places, layers of trachytic tuff are interstratified, and in these tuffs are leaves of plants identical with those found in the brown-coal, showing that, during the period of the accumulation of the latter, some volcanic products were ejected.
The varieties of wood in the lignite are said to belong entirely to dicotyledonous trees; but among the impressions of leaves, collected by Mr. Horner, some were referred by Mr. Lindley to a palm, perhaps of the genusChamærops, and others resembled theCinnamomum dulce, andPodocarpus macrophylla, which would also indicate a warm climate.[416-A]
The other organic remains of the brown-coal are principally fishes; they are found in a bituminous shale, called paper-coal, from beingdivisible into extremely thin leaves. The individuals are very numerous; but they appear to belong to about five species, which M. Agassiz informs me are all extinct, and hitherto peculiar to this brown-coal. They belong to the freshwater generaLeuciscus,Aspius, andPerca. The remains of frogs also, of an extinct species, have been discovered in the paper-coal; and a complete series may be seen in the museum at Bonn, from the most imperfect state of the tadpole to that of the full-grown animal. With these a salamander, scarcely distinguishable from the recent species, has been found, and several remains of insects.
The brown-coal was evidently a freshwater formation; but fossil shells have been scarcely ever found in it; although near Marienforst, in the vicinity of Bonn, large blocks have been met with of a white opaque chert, containing numerous casts of freshwater shells, which appear to belong toPlanorbis rotundatusandLimnea longiscata, two species common both to the Middle and Upper Eocene periods. It is very probable that the brown-coal may be connected in age with those fluvio-marine formations which are found in higher parts of the valley of the Rhine, as at Mayence before mentioned (p. 177.).
A vast deposit of gravel, chiefly composed of pebbles of white quartz, but containing also a few fragments of other rocks, lies over the brown-coal formation, forming sometimes only a thin covering, at others attaining a thickness of more than 100 feet. This gravel is very distinct in character from that now forming the bed of the Rhine. It is called "Kiesel gerolle" by the Germans, often reaches great elevations, and is covered in several places with volcanic ejections. It is evident that the country has undergone great changes in its physical geography since this gravel was formed; for its position has scarcely any relation to the existing drainage of the country, and all the more modern volcanic rocks of the same region are posterior to it in date.
Some of the newest beds of volcanic sand, pumice, and scoriæ are interstratified near Andernach and elsewhere with the loam called loess, which was before described as being full of land and freshwater shells of recent species, and referable to the Post-Pliocene period. I have before hinted (seep. 118.) that this intercalation of volcanic matter between beds of loess may possibly be explained without supposing the last eruptions of the Lower Eifel to have taken place so recently as the era of the deposition of the loess; but farther researches should be directed to the investigation of this curious point.
The igneous rocks of the Westerwald, and of the mountains called the Siebengebirge, consist partly of basaltic and partly of trachytic lavas, the latter being in general the more ancient of the two. There are many varieties of trachyte, some of which are highly crystalline, resembling a coarse-grained granite, with large separate crystals of felspar. Trachytic tuff is also very abundant. These formations, some of which were certainly contemporaneous with the origin of the brown-coal, were the first of a long series of eruptions, themore recent of which happened when the country had acquired nearly all its present geographical features.
Newer volcanos of the Eifel.—Lake-craters.—As I recognized in the more modern volcanos of the Eifel characters distinct from any previously observed by me in those of France, Italy, or Spain, I shall briefly describe them. The fundamental rocks of the district are grey and red sandstones and shales, with some associated limestones, replete with fossils of the Devonian or Old Red Sandstone group. The volcanos broke out in the midst of these inclined strata, and when the present systems of hills and valleys had already been formed. The eruptions occurred sometimes at the bottom of deep valleys, sometimes on the summit of hills, and frequently on intervening platforms. In travelling through this district we often fall upon them most unexpectedly, and may find ourselves on the very edge of a crater before we had been led to suspect that we were approaching the site of any igneous outburst. Thus, for example, on arriving at the village of Gemund, immediately south of Daun, we leave the stream, which flows at the bottom of a deep valley in which strata of sandstone and shale crop out. We then climb a steep hill, on the surface of which we see the edges of the same strata dipping inwards towards the mountain. When we have ascended to a considerable height, we see fragments of scoriæ sparingly scattered over the surface; till, at length, on reaching the summit, we find ourselves suddenly on the edge of atarn, or deep circular lake-basin.
Fig. 477.The Gemunder Maar.
Fig. 477.
The Gemunder Maar.
Fig. 478.a.Village of Gemund.b.Gemunder Maar.c.Weinfelder Maar.d.Schalkenmehren Maar.
Fig. 478.
This, which is called the Gemunder Maar, is the first of three lakes which are in immediate contact, the same ridge forming the barrier of two neighbouring cavities (seefig. 477.). On viewing the first of these, we recognize the ordinary form of a crater, for whichwe have been prepared by the occurrence of scoriæ scattered over the surface of the soil. But on examining the walls of the crater we find precipices of sandstone and shale which exhibit no signs of the action of heat; and we look in vain for those beds of lava and scoriæ, dipping in opposite directions on every side, which we have been accustomed to consider as characteristic of volcanic craters. As we proceed, however, to the opposite side of the lake, and afterwards visit the craterscandd(fig. 478.), we find a considerable quantity of scoriæ and some lava, and see the whole surface of the soil sparkling with volcanic sand, and strewed with ejected fragments of half-fused shale, which preserves its laminated texture in the interior, while it has a vitrified or scoriform coating.
A few miles to the south of the lakes above mentioned occurs the Pulvermaar of Gillenfeld, an oval lake of very regular form, and surrounded by an unbroken ridge of fragmentary materials, consisting of ejected shale and sandstone, and preserving a uniform height of about 150 feet above the water. The side slope in the interior is at an angle of about 45 degrees; on the exterior, of 35 degrees. Volcanic substances are intermixed very sparingly with the ejections, which in this place entirely conceal from view the stratified rocks of the country.[419-A]
Fig. 479.Outline of Mosenberg, Upper Eifel.
Fig. 479.
Outline of Mosenberg, Upper Eifel.
The Meerfelder Maar is a cavity of far greater size and depth, hollowed out of similar strata; the sides presenting some abrupt sections of inclined secondary rocks, which in other places are buried under vast heaps of pulverized shale. I could discover no scoriæ amongst the ejected materials, but balls of olivine and other volcanic substances are mentioned as having been found.[419-B]This cavity, which we must suppose to have discharged an immense volume of gas, is nearly a mile in diameter, and is said to be more than one hundred fathoms deep. In the neighbourhood is a mountain called the Mosenberg, which consists of red sandstone and shale in its lower parts, but supports on its summit a triple volcanic cone, while a distinct current of lava is seen descending the flanks of the mountain. The edge of the crater of the largest cone reminded me much of the form and characters of that of Vesuvius; but I was much struck with the precipitous and almost overhanging wall or parapet which the scoriæ presented towards the exterior, as ata b(fig. 479.); which I can only explain by supposing that fragments of red-hot lava, as they fellround the vent, were cemented together into one compact mass, in consequence of continuing to be in a half-melted state.
If we pass from the Upper to the Lower Eifel, fromAtoB(see map,p. 416.), we find the celebrated lake-crater of Laach, which has a greater resemblance than any of those before mentioned to the Lago di Bolsena, and others in Italy—being surrounded by a ridge of gently sloping hills, composed of loose tuffs, scoriæ, and blocks of a variety of lavas.
One of the most interesting volcanos on the left bank of the Rhine is called the Roderberg. It forms a circular crater nearly a quarter of a mile in diameter, and 100 feet deep, now covered with fields of corn. The highly inclined strata of ancient sandstone and shale rise even to the rim of one side of the crater; but they are overspread by quartzose gravel, and this again is covered by volcanic scoriæ and tufaceous sand. The opposite wall of the crater is composed of cinders and scorified rock, like that at the summit of Vesuvius. It is quite evident that the eruption in this case burst through the sandstone and alluvium which immediately overlies it; and I observed some of the quartz pebbles mixed with scoriæ on the flanks of the mountain, as if they had been cast up into the air, and had fallen again with the volcanic ashes. I have already observed, that a large part of this crater has been filled up with loess (p. 118.).
The most striking peculiarity of a great many of the craters above described, is the absence of any signs of alteration or torrefaction in their walls, when these are composed of regular strata of ancient sandstone and shale. It is evident that the summits of hills formed of the above-mentioned stratified rocks have, in some cases, been carried away by gaseous explosions, while at the same time no lava, and often a very small quantity only of scoriæ, has escaped from the newly formed cavity. There is, indeed, no feature in the Eifel volcanos more worthy of note, than the proofs they afford of very copious aëriform discharges, unaccompanied by the pouring out of melted matter, except, here and there, in very insignificant volume. I know of no other extinct volcanos where gaseous explosions of such magnitude have been attended by the emission of so small a quantity of lava. Yet I looked in vain in the Eifel for any appearances which could lend support to the hypothesis, that the sudden rushing out of such enormous volumes of gas had ever lifted up the stratified rocks immediately around the vent, so as to form conical masses, having their strata dipping outwards on all sides from a central axis, as is assumed in the theory of elevation craters, alluded to at the end of Chap. XXIX.
Trass.—In the Lower Eifel, eruptions of trachytic lava preceded the emission of currents of basalt, and immense quantities of pumice were thrown out wherever trachyte issued. The tufaceous alluvium calledtrass, which has covered large areas in this region and choked up some valleys now partially re-excavated, is unstratified. Its base consists almost entirely of pumice, in which are included fragments of basalt and other lavas, pieces of burnt shale, slate, and sandstone,and numerous trunks and branches of trees. If this trass was formed during the period of volcanic eruptions it may perhaps have originated in the manner of the moya of the Andes.
We may easily conceive that a similar mass might now be produced, if a copious evolution of gases should occur in one of the lake basins. The water might remain for weeks in a state of violent ebullition, until it became of the consistency of mud, just as the sea continued to be charged with red mud round Graham's Island, in the Mediterranean, in the year 1831. If a breach should then be made in the side of the cone, the flood would sweep away great heaps of ejected fragments of shale and sandstone, which would be borne down into the adjoining valleys. Forests might be torn up by such a flood, and thus the occurrence of the numerous trunks of trees dispersed irregularly through the trass, can be explained.
Hungary.—M. Beudant, in his elaborate work on Hungary, describes five distinct groups of volcanic rocks, which although nowhere of great extent, form striking features in the physical geography of that country, rising as they do abruptly from extensive plains composed of tertiary strata. They may have constituted islands in the ancient sea, as Santorin and Milo now do in the Grecian Archipelago; and M. Beudant has remarked that the mineral products of the last-mentioned islands resemble remarkably those of the Hungarian extinct volcanos, where many of the same minerals as opal, calcedony, resinous silex (silex resinite), pearlite, obsidian, and pitchstone abound.
The Hungarian lavas are chiefly felspathic, consisting of different varieties of trachyte; many are cellular, and used as millstones; some so porous and even scoriform as to resemble those which have issued in the open air. Pumice occurs in great quantity; and there are conglomerates, or rather breccias, wherein fragments of trachyte are bound together by pumiceous tuff, or sometimes by silex.
It is probable that these rocks were permeated by the waters of hot springs, impregnated, like the Geysers, with silica; or in some instances, perhaps, by aqueous vapours, which, like those of Lancerote, may have precipitated hydrate of silica.
By the influence of such springs or vapours the trunks and branches of trees washed down during floods, and buried in tuffs on the flanks of the mountains, are supposed to have become silicified. It is scarcely possible, says M. Beudant, to dig into any of the pumiceous deposits of these mountains without meeting with opalized wood, and sometimes entire silicified trunks of trees of great size and weight.
It appears from the species of shells collected principally by M. Boué, and examined by M. Deshayes, that the fossil remains imbedded in the volcanic tuffs, and in strata alternating with them in Hungary, are of the Miocene type, and not identical, as was formerly supposed, with the fossils of the Paris basin.
Volcanic rocks of the Pliocene and Miocene periods continued — Auvergne — Mont Dor — Breccias and alluviums of Mont Perrier, with bones of quadrupeds — River dammed up by lava-current — Range of minor cones from Auvergne to the Vivarais — Monts Dome — Puy de Côme — Puy de Pariou — Cones not denuded by general flood — Velay — Bones of quadrupeds buried in scoriæ — Cantal — Eocene volcanic rocks — Tuffs near Clermont — Hill of Gergovia — Trap of Cretaceous period — Oolitic period — New Red Sandstone period — Carboniferous period — Old Red Sandstone period — "Rock and Spindle" near St. Andrews — Silurian period — Cambrian volcanic rocks.
Volcanic rocks of the Pliocene and Miocene periods continued — Auvergne — Mont Dor — Breccias and alluviums of Mont Perrier, with bones of quadrupeds — River dammed up by lava-current — Range of minor cones from Auvergne to the Vivarais — Monts Dome — Puy de Côme — Puy de Pariou — Cones not denuded by general flood — Velay — Bones of quadrupeds buried in scoriæ — Cantal — Eocene volcanic rocks — Tuffs near Clermont — Hill of Gergovia — Trap of Cretaceous period — Oolitic period — New Red Sandstone period — Carboniferous period — Old Red Sandstone period — "Rock and Spindle" near St. Andrews — Silurian period — Cambrian volcanic rocks.
TertiaryVolcanic Rocks.—Auvergne.—The extinct volcanos of Auvergne and Cantal in Central France seem to have commenced their eruptions in the Upper Eocene period, but to have been most active during the Miocene and Pliocene eras. I have already alluded to the grand succession of events, of which there is evidence in Auvergne since the last retreat of the sea (seep. 178.).
The earliest monuments of the tertiary period in that region are lacustrine deposits of great thickness (2.fig. 480.p. 424.), in the lowest conglomerates of which are rounded pebbles of quartz, mica-schist, granite, and other non-volcanic rocks, without the slightest intermixture of igneous products. To these conglomerates succeed argillaceous and calcareous marls and limestones (3.fig. 480.) containing Upper Eocene shells and bones of mammalia, the higher beds of which sometimes alternate with volcanic tuff of contemporaneous origin. After the filling up or drainage of the ancient lakes, huge piles of trachytic and basaltic rocks, with volcanic breccias, accumulated to a thickness of several thousand feet, and were superimposed upon granite, or the contiguous lacustrine strata. The greater portion of these igneous rocks appear to have originated during the Miocene and Pliocene periods; and extinct quadrupeds of those eras, belonging to the genera Mastodon, Rhinoceros, and others, were buried in ashes and beds of alluvial sand and gravel, which owe their preservation to overspreading sheets of lava.
In Auvergne the most ancient and conspicuous of the volcanic masses is Mont Dor, which rests immediately on the granitic rocks standing apart from the freshwater strata.[422-A]This great mountain rises suddenly to the height of several thousand feet above the surrounding platform, and retains the shape of a flattened and somewhat irregular cone, all the sides sloping more or less rapidly, until their inclination is gradually lost in the high plain around. This cone is composed of layers of scoriæ, pumice-stones, and their fine detritus,with interposed beds of trachyte and basalt, which descend often in uninterrupted sheets, till they reach and spread themselves round the base of the mountain.[423-A]Conglomerates, also, composed of angular and rounded fragments of igneous rocks, are observed to alternate with the above; and the various masses are seen to dip off from the central axis, and to lie parallel to the sloping flanks of the mountain.
The summit of Mont Dor terminates in seven or eight rocky peaks, where no regular crater can now be traced, but where we may easily imagine one to have existed, which may have been shattered by earthquakes, and have suffered degradation by aqueous agents. Originally, perhaps, like the highest crater of Etna, it may have formed an insignificant feature in the great pile, and may frequently have been destroyed and renovated.
According to some geologists, this mountain, as well as Vesuvius, Etna, and all large volcanos, has derived its dome-like form not from the preponderance of eruptions from one or more central points, but from the upheaval of horizontal beds of lava and scoriæ. I have explained my reasons for objecting to this view at the close of Chap. XXIX., when speaking of Palma, and in the Principles of Geology.[423-B]The average inclination of the dome-shaped mass of Mont Dor is 8° 6', whereas in Mounts Loa and Kea, before mentioned, in the Sandwich Islands (seefig. 457.p. 394.), the flanks of which have been raised by recent lavas, we find from Mr. Dana's description that the one has a slope of 6° 30', the other of 7° 46'. We may, therefore, reasonably question whether there is any absolute necessity for supposing that the basaltic currents of the ancient French volcano were at first more horizontal than they are now. Nevertheless it is highly probable that during the long series of eruptions required to give rise to so vast a pile of volcanic matter, which is thickest at the summit or centre of the dome, some dislocation and upheaval took place; and during the distension of the mass, beds of lava and scoriæ may, in some places, have acquired a greater, in others a less inclination, than that which at first belonged to them.
Respecting the age of the great mass of Mont Dor, we cannot come at present to any positive decision, because no organic remains have yet been found in the tuffs, except impressions of the leaves of trees of species not yet determined. We may certainly conclude, that the earliest eruptions were posterior in origin to those grits, and conglomerates of the freshwater formation of the Limagne, which contain no pebbles of volcanic rocks; while, on the other hand, some eruptions took place before the great lakes were drained; and others occurred after the desiccation of those lakes, and when deep valleys had already been excavated through freshwater strata.
In the annexed section, I have endeavoured to explain the geological structure of a portion of Auvergne, which I re-examined in 1843.[423-C]It may convey some idea to the reader of the long and complicated series of events, which have occurred in that country, since the first lacustrine strata (No. 2.) were deposited on the granite (No. 1.). The changes of which we have evidence are the more striking, because they imply great denudation, without there being any proofs of the intervention of the sea during the whole period. It will be seen that the upper freshwater beds (No. 3.), once formed in a lake, must have suffered great destruction before the excavation of the valleys of the Couze and Allier had begun. In these freshwater beds, Upper Eocene fossils, as described in Chap. XV., have been found. The basaltic dike 4' is one of many examples of the intrusion of volcanic matter through the Eocene freshwater beds, and may have been of Upper Eocene or Miocene date, giving rise, when it reached the surface and overflowed, to such platforms of basalt, as often cap the tertiary hills in Auvergne, and one of which (4) is seen on Mont Perrier.
Fig. 480.Section from the valley of the Couze at Nechers, through Mont Perrier and Issoire to the Valley of the Allier, and the Tour de Boulade, Auvergne.10. Lava-current of Tartaret near its termination at Nechers.9. Bone-bed, red sandy clay under the lava of Tartaret.8. Bone-bed of the Tour de Boulade.7. Alluvium newer than No. 6.6. Alluvium with bones of hippopotamus.5c.Trachytic breccia resembling 5a.5b.Upper bone-bed of Perrier, gravel, &c.5a.Pumiceous breccia and conglomerate, angular masses of trachyte, quartz, pebbles, &c.5. Lower bone-bed of Perrier, ochreous sand and gravel.4a.Basaltic dyke.4. Basaltic platform.3. Upper freshwater beds, limestone, marl, gypsum, &c.2. Lower freshwater formation, red clay, green sand, &c.1. Granite.
Fig. 480.
Section from the valley of the Couze at Nechers, through Mont Perrier and Issoire to the Valley of the Allier, and the Tour de Boulade, Auvergne.
It not unfrequently happens that beds of gravel containing bones of extinct mammalia are detected under these very ancient sheets of basalt, as between No. 4. and the freshwater strata, No. 3., atA, from which it is clear that the surface of 3 formed at that period the lowest level at which the waters then draining the country flowed. Next in age to this basaltic platform comes a patch of ochreous sand and gravel (No. 5.), containing many bones of quadrupeds. Upon this rests a pumiceous breccia and conglomerate, with angular masses of trachyte, and some quartz pebbles. This deposit is followed by 5b, which is similar to 5, and 5csimilar to the trachytic breccia 5a. These two breccias are supposed, from their similarity to others found on Mount Dor, to have descended from the flanks of that mountain during eruptions; and the interstratified alluvial deposits contain the remains of mastodon, rhinoceros, tapir, deer, beaver, and quadrupeds of other genera referable to about forty species, all of which are extinct. I formerly supposed them to belong to the same era as the Miocene faluns of Touraine; but, whether they may not rather beascribed to the older Pliocene epoch is a question which farther inquiries and comparisons must determine.
Whatever be their date in the tertiary series, they are quadrupeds which inhabited the country when the formations 5 and 5coriginated. Probably they were drowned during floods, such as rush down the flanks of volcanos during eruptions, when great bodies of steam are emitted from the crater, or when, as we have seen, both on Etna and in Iceland in modern times, large masses of snow are suddenly melted by lava, causing a deluge of water to bear down fragments of igneous rocks mixed with mud, to the valleys and plains below.
It will be seen that the valley of the Issoire, down which these ancient inundations swept, was first excavated at the expense of the formations 2, 3, and 4, and then filled up by the masses 5 and 5c, after which it was re-excavated before the more modern alluviums (Nos. 6. and 7.) were formed. In these again other fossil mammalia of distinct species have been detected by M. Bravard, the bones of an hippopotamus having been found among the rest.
At length, when the valley of the Allier was eroded at Issoire down to its lowest level, a talus of angular fragments of basalt and freshwater limestone (No. 8.) was formed, called the bone-bed of the Tour de Boulade, from which a great many other mammalia have been collected by MM. Bravard and Pomel. In this assemblage theElephas primigenius,Rhinoceros tichorinus,Deer(including rein-deer),Equus,Bos,Antelope,Felis, andCanis, were included. Even this deposit seems hardly to be the newest in the neighbourhood, for if we cross from the town of Issoire (seefig. 480.) over Mont Perrier to the adjoining valley of the Couze, we find another bone-bed (No. 9.), overlaid by a current of lava (No. 10.).
The history of this lava-current, which terminates a few hundred yards below the point No. 10., in the suburbs of the village of Nechers, is interesting. It forms a long narrow stripe more than 13 miles in length, at the bottom of the valley of the Couze, which flows out of a lake at the foot of Mont Dor. This lake is caused by a barrier thrown across the ancient channel of the Couze, consisting partly of the volcanic cone called the Puy de Tartaret, formed of loose scoriæ, from the base of which has issued the lava-current before mentioned. The materials of the dam which blocked up the river, and caused the Lac de Chambon, are also, in part, derived from a land-slip which may have happened at the time of the great eruption which formed the cone.
This cone of Tartaret affords an impressive monument of the very different dates at which the igneous eruptions of Auvergne have happened; for it was evidently thrown up at the bottom of the existing valley, which is bounded by lofty precipices composed of sheets of ancient columnar trachyte and basalt, which once flowed at very high levels from Mont Dor.[425-A]
When we follow the course of the river Couze, from its source in the lake of Chambon, to the termination of the lava-current at Nechers, a distance of thirteen miles, we find that the torrent has in most places cut a deep channel through the lava, the lower portion of which is columnar. In some narrow gorges it has even had power to remove the entire mass of basaltic rock, though the work of erosion must have been very slow, as the basalt is tough and hard, and one column after another must have been undermined and reduced to pebbles, and then to sand. During the time required for this operation, the perishable cone of Tartaret, composed of sand and ashes, has stood uninjured, proving that no great flood or deluge can have passed over this region in the interval between the eruption of Tartaret and our own times.
If we now return to the section (fig. 480.), I may observe that the lava-current of Tartaret, which has diminished greatly in height and volume near its termination, presents here a steep and perpendicular face 25 feet in height towards the river. Beneath it is the alluvium No. 9., consisting of a red sandy clay, which must have covered the bottom of the valley when the current of melted rock flowed down. The bones found in this alluvium, which I obtained myself, consisted of a species of field-mouse,Arvicola, and the molar tooth of an extinct horse,Equus fossilis. The other species, obtained from the same bed, are referable to the generaSus,Bos,Cervus,Felis,Canis,Martes,Talpa,Sorex,Lepus,Sciurus,Mus, andLagomys, in all no less than forty-three species, all closely allied to recent animals, yet nearly all of them, according to M. Bravard, showing some points of difference, like those which Mr. Owen discovered in the case of the horse above alluded to. The bones, also, of a frog, snake, and lizard, and of several birds, were associated with the fossils before enumerated, and several recent land shells, such asCyclostoma elegans,Helix hortensis,H. nemoralis,H. lapicida, andClausilia rugosa. If the animals were drowned by floods, which accompanied the eruptions of the Puy de Tartaret, they would give an exceedingly modern geological date to that event, which must, in that case, have belonged to the Newer-Pliocene, or, perhaps, the Post-Pliocene period. That the current, which has issued from the Puy de Tartaret, may nevertheless be very ancient in reference to the events of human history, we may conclude, not only from the divergence of the mammiferous fauna from that of our day, but from the fact that a Roman bridge of such form and construction as continued in use down to the fifth century, but which may be older, is now seen at a place about a mile and a half from St. Nectaire. This ancient bridge spans the river Couze with two arches, each about 14 feet wide. These arches spring from the lava of Tartaret, on both banks, showing that a ravine precisely like that now existing, had already been excavated by the river through that lava thirteen or fourteen centuries ago.
In Central France there are several hundred minor cones, like that of Tartaret, a great number of which, like Monte Nuovo, near Naples, may have been principally due to a single eruption. Most of thesecones range in a linear direction from Auvergne to the Vivarais, and they were faithfully described so early as the year 1802, by M. de Montlosier. They have given rise chiefly to currents of basaltic lava. Those of Auvergne called the Monts Dome, placed on a granitic platform, form an irregular ridge (seefig. 436.), about 18 miles in length, and 2 in breadth. They are usually truncated at the summit, where the crater is often preserved entire, the lava having issued from the base of the hill. But frequently the crater is broken down on one side, where the lava has flowed out. The hills are composed of loose scoriæ, blocks of lava, lapilli, and pozzuolana, with fragments of trachyte and granite.
Puy de Côme.—The Puy de Côme and its lava-current, near Clermont, may be mentioned as one of these minor volcanos. This conical hill rises from the granitic platform, at an angle of about 40°, to the height of more than 900 feet. Its summit presents two distinct craters, one of them with a vertical depth of 250 feet. A stream of lava takes its rise at the western base of the hill, instead of issuing from either crater, and descends the granitic slope towards the present site of the town of Pont Gibaud. Thence it pours in a broad sheet down a steep declivity into the valley of the Sioule, filling the ancient river-channel for the distance of more than a mile. The Sioule, thus dispossessed of its bed, has worked out a fresh one between the lava and the granite of its western bank; and the excavation has disclosed, in one spot, a wall of columnar basalt about 50 feet high.[427-A]
The excavation of the ravine is still in progress, every winter some columns of basalt being undermined and carried down the channel of the river, and in the course of a few miles rolled to sand and pebbles. Meanwhile the cone of Côme remains stationary, its loose materials being protected by a dense vegetation, and the hill standing on a ridge not commanded by any higher ground whence floods of rain-water may descend.
Puy Rouge.—At another point, farther down the course of the Sioule, we find a second illustration of the same phenomenon in the Puy Rouge, a conical hill to the north of the village of Pranal. The cone is composed entirely of red and black scoriæ, tuff, and volcanic bombs. On its western side there is a worn-down crater, whence a powerful stream of lava has issued, and flowed into the valley of the Sioule. The river has since excavated a ravine through the lava and subjacent gneiss, to the depth of 400 feet.
On the upper part of the precipice forming the left side of this ravine, we see a great mass of black and red scoriaceous lava; below this a thin bed of gravel, evidently an ancient river-bed, now at an elevation of 50 feet above the channel of the Sioule. The gravel again rests upon gneiss, which has been eroded to the depth of 50 feet. It is quite evident in this case, that, while the basalt was gradually undermined and carried away by the force of running water, the cone whence the lava issued escaped destruction, because it stoodupon a platform of gneiss several hundred feet above the level of the valley in which the force of running water was exerted.
Puy de Pariou.—The brim of the crater of the Puy de Pariou, near Clermont, is so sharp, and has been so little blunted by time, that it scarcely affords room to stand upon. This and other cones in an equally remarkable state of integrity have stood, I conceive uninjured, notin spiteof their loose porous nature, as might at first be naturally supposed, but in consequence of it. No rills can collect where all the rain is instantly absorbed by the sand and scoriæ, as is remarkably the case on Etna; and nothing but a waterspout breaking directly upon the Puy de Pariou could carry away a portion of the hill, so long as it is not rent or engulphed by earthquakes.
Hence it is conceivable that even those cones which have the freshest aspect, and most perfect shape, may lay claim to very high antiquity. Dr. Daubeny has justly observed, that had any of these volcanos been in a state of activity in the age of Julius Cæsar, that general, who encamped upon the plains of Auvergne, and laid siege to its principal city (Gergovia, near Clermont), could hardly have failed to notice them. Had there been any record of their eruptions in the time of Pliny or Sidonius Apollinaris, the one would scarcely have omitted to make mention of it in his Natural History, nor the other to introduce some allusion to it among the descriptions of this his native province. This poet's residence was on the borders of the Lake Aidat, which owed its very existence to the damming up of a river by one of the most modern lava-currents.[428-A]
Velay.—The observations of M. Bertrand de Doue have not yet established that any of the most ancient volcanos of Velay were in action during the Eocene period. There are beds of gravel in Velay, as in Auvergne, covered by lava at different heights above the channels of the existing rivers. In the highest and most ancient of these alluviums the pebbles are exclusively of granitic rocks; but in the newer, which are found at lower levels, and which originated when the valleys had been cut to a greater depth, an intermixture of volcanic rocks has been observed.
At St. Privat d'Allier a bed of volcanic scoriæ and tuff was discovered by Dr. Hibbert, inclosed between two sheets of basaltic lava; and in this tuff were found the bones of several quadrupeds, some of them adhering to masses of slaggy lava. Among other animals wereRhinoceros leptorhinus,Hyæna spelæa, and a species allied to the spotted hyæna of the Cape, together with four undetermined species of deer.[428-B]The manner of the occurrence of these bones reminds us of the published accounts of an eruption of Coseguina, 1835, in Central America (seep. 399.), during which hot cinders and scoriæ fell and scorched to death great numbers of wild and domestic animals and birds.
Plomb du Cantal.—In regard to the age of the igneous rocks ofthe Cantal, we can at present merely affirm, that they overlie the Eocene lacustrine strata of that country (see Map,p. 179.). They form a great dome-shaped mass, having an average slope of only 4°, which has evidently been accumulated, like the cone of Etna, during a long series of eruptions. It is composed of trachytic, phonolitic, and basaltic lavas, tuffs, and conglomerates, or breccias, forming a mountain several thousand feet in height. Dikes also of phonolite, trachyte, and basalt are numerous, especially in the neighbourhood of the large cavity, probably once a crater, around which the loftiest summits of the Cantal are ranged circularly, few of them, except the Plomb du Cantal, rising far above the border or ridge of this supposed crater. A pyramidal hill, called the Puy Griou, occupies the middle of the cavity.[429-A]It is clear that the volcano of the Cantal broke out precisely on the site of the lacustrine deposit before described (p. 188.), which had accumulated in a depression of a tract composed of micaceous schist. In the breccias, even to the very summit of the mountain, we find ejected masses of the freshwater beds, and sometimes fragments of flint, containing Eocene shells. Valleys radiate in all directions from the central heights of the mountain, increasing in size as they recede from those heights. Those of the Cer and Jourdanne, which are more than 20 miles in length, are of great depth, and lay open the geological structure of the mountain. No alternation of lavas with undisturbed Eocene strata has been observed, nor any tuffs containing freshwater shells, although some of these tuffs include fossil remains of terrestrial plants, said to imply several distinct restorations of the vegetation of the mountain in the intervals between great eruptions. On the northern side of the Plomb du Cantal, at La Vissiere, near Murat, is a spot, pointed out on the Map (p. 179.), where freshwater limestone and marl are seen covered by a thickness of about 800 feet of volcanic rock. Shifts are here seen in the strata of limestone and marl.[429-B]
Eocene period.—In treating of the lacustrine deposits of Central France, in the fifteenth chapter, it was stated that, in the arenaceous and pebbly group of the lacustrine basins of Auvergne, Cantal, and Velay, no volcanic pebbles had ever been detected, although massive piles of igneous rocks are now found in the immediate vicinity. As this observation has been confirmed by minute research, we are warranted in inferring that the volcanic eruptions had not commenced when the older subdivisions of the freshwater groups originated.
In Cantal and Velay no decisive proofs have yet been brought to light that any of the igneous outbursts happened during the deposition of the freshwater strata; but there can be no doubt that in Auvergne some volcanic explosions took place before the drainage of the lakes, and at a time when the Upper Eocene species of animals and plants still flourished. Thus, for example, at Pont du Chateau, near Clermont, a section is seen in a precipice on the right bank ofthe river Allier, in which beds of volcanic tuff alternate with a freshwater limestone, which is in some places pure, but in others spotted with fragments of volcanic matter, as if it were deposited while showers of sand and scoriæ were projected from a neighbouring vent.[430-A]
Another example occurs in the Puy de Marmont, near Veyres, where a freshwater marl alternates with volcanic tuff containing Eocene shells. The tuff or breccia in this locality is precisely such as is known to result from volcanic ashes falling into water, and subsiding together with ejected fragments of marl and other stratified rocks. These tuffs and marls are highly inclined, and traversed by a thick vein of basalt, which, as it rises in the hill, divides into two branches.
Gergovia.—The hill of Gergovia, near Clermont, affords a third example. I agree with MM. Dufrénoy and Jobert that there is no alternation here of a contemporaneous sheet of lava with freshwater strata, in the manner supposed by some other observers[430-B]; but the position and contents of some of the associated tuffs, prove them to have been derived from volcanic eruptions which occurred during the deposition of the lacustrine strata.