A Manual of Elementary Geology Part 65

_c._ Weinfelder Maar.

_d._ Schalkenmehren Maar.]

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 (see fig. 477.). On viewing the first of these, we recognize the ordinary form of a crater, for which we have been prepared by the occurrence of scoriae 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 scoriae, 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 craters _c_ and _d_ (fig. 478.), we find a considerable quant.i.ty of scoriae 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]

[Ill.u.s.tration: 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 scoriae amongst the ejected materials, but b.a.l.l.s 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 scoriae presented towards the exterior, as at _a b_ (fig. 479.); which I can only explain by supposing that fragments of red-hot lava, as they fell round the vent, were cemented together into one compact ma.s.s, in consequence of continuing to be in a half-melted state.

If we pa.s.s from the Upper to the Lower Eifel, from A to B (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, scoriae, 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 scoriae 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 scoriae 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 quant.i.ty only of scoriae, 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 aeriform 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 quant.i.ty of lava. Yet I looked in vain in the Eifel for any appearances which could lend support to the hypothesis, that the sudden rus.h.i.+ng out of such enormous volumes of gas had ever lifted up the stratified rocks immediately around the vent, so as to form conical ma.s.ses, having their strata dipping outwards on all sides from a central axis, as is a.s.sumed in the theory of elevation craters, alluded to at the end of Chap. XXIX.

_Tra.s.s._--In the Lower Eifel, eruptions of trachytic lava preceded the emission of currents of basalt, and immense quant.i.ties of pumice were thrown out wherever trachyte issued. The tufaceous alluvium called _tra.s.s_, 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 tra.s.s 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 ma.s.s 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 tra.s.s, 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 const.i.tuted 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 quant.i.ty; 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 sh.e.l.ls collected princ.i.p.ally by M. Boue, 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.

FOOTNOTES:

[409-A] Maclure, Journ. de Phys., vol. lxvi. p. 219., 1808; cited by Daubeny, Description of Volcanos, p. 24.

[410-A] This view is taken from a sketch which I made on the spot in 1830.

[416-A] Trans. of Geol. Soc., 2d series, vol. v.

[419-A] Scrope, Edin. Journ. of Sci., June, 1826, p. 145.

[419-B] Hibbert, Extinct Volcanos of the Rhine, p. 24.

CHAPTER x.x.xII.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.

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 Come--Puy de Pariou--Cones not denuded by general flood--Velay--Bones of quadrupeds buried in scoriae--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.

_Tertiary Volcanic 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 (see p. 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 sh.e.l.ls 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, acc.u.mulated 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 ma.s.ses 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 scoriae, 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 ma.s.ses 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 scoriae. 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 ma.s.s of Mont Dor is 8 6', whereas in Mounts Loa and Kea, before mentioned, in the Sandwich Islands (see fig. 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 ma.s.s, beds of lava and scoriae 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 ma.s.s 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.

[Ill.u.s.tration: 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.

5 _c._ Trachytic breccia resembling 5 _a._ 5 _b._ Upper bone-bed of Perrier, gravel, &c.

5 _a._ Pumiceous breccia and conglomerate, angular ma.s.ses of trachyte, quartz, pebbles, &c.

5. Lower bone-bed of Perrier, ochreous sand and gravel.

4 _a._ Basaltic d.y.k.e.

4. Basaltic platform.

3. Upper freshwater beds, limestone, marl, gypsum, &c.

2. Lower freshwater formation, red clay, green sand, &c.

1. Granite.]

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., at A, 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 ma.s.ses of trachyte, and some quartz pebbles. This deposit is followed by 5 _b_, which is similar to 5, and 5 _c_ similar to the trachytic breccia 5 _a_. 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 be ascribed 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 5 _c_ originated.

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 ma.s.ses 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 ma.s.ses 5 and 5 _c_, 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 a.s.semblage the _Elephas primigenius_, _Rhinoceros tichorinus_, _Deer_ (including rein-deer), _Equus_, _Bos_, _Antelope_, _Felis_, and _Canis_, were included. Even this deposit seems hardly to be the newest in the neighbourhood, for if we cross from the town of Issoire (see fig. 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 scoriae, 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]