Part 65 (1/2)
_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.