CHAPTER XXXII.
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 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.
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 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 (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 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.