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., 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 masses 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 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 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 assemblage 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 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.