Ch. X.V.] LACUSTRINE STRATA — AUVERGNE. 197 



flows of mud, sucli as accompany volcanic eruptions. Lastly, these quad- 

 rupeds became extinct, and gave place to Pliocene mammalia (see chap, 

 xxxii.), and these in their turn, to species now existing. There are no 

 signs, during the whole time required for this series of events, of the sea 

 having intervened, nor of any denudation which may not have been ac- 

 complished by currents in the different lakes, or by rivers and floods ac- 

 companying repeated earthquakes, during which the levels of the district 

 have in some places been materially modified, and perhaps the whole up- 

 raised relatively to the surrounding parts of France. 



Auvergne. — The most northern of the freshwater groups is situated in 

 the valley-plain of the Allier, which lies within the department of the Puy 

 de Dome, being the tract which went formerly by the name of the Li- 

 magne d'Auvei'gue. It is inclosed by two parallel mountain ranges, — 

 that of the Forez, which divides the waters of the Loire and Allier, on 

 the east ; and that of the Monts Domes, which separates the Allier from 

 the Sioule, on the west."^ The average breadth of this tract is about 20 

 miles ; and it is for the most part composed of nearly horizontal strata of 

 sand, sandstone, calcareous marl, clay, and limestone, none of v,'hich ob- 

 serve a fixed and invariable order of superposition. The ancient borders 

 of the lake, wherein the freshwater strata were accumulated, may gen- 

 erally be traced with precision, the granite and other ancient rocks rising 

 up boldly from the level country. The actual junction, however, of the 

 lacustrine and granitic beds is rarely seen, as a small valley usually in- 

 tervenes between them. The freshwater strata may sometimes be seen 

 to retain their horizontality within a very slight distance of the border- 

 rocks, while in some places they are inclined, and in few instances vertical. 

 The principal divisions into which the lacustrine series may be separated 

 are the following ; — 1st, Sandstone, grit, and conglomerate, including red 

 marl and red sandstone, 2dly, Green and white foliated marls. 3dly, 

 Limestone or travertin, often oolitic. 4thly, Gypseous marls. 



1. a. Sandstone and conglomerate. — Strata of sand and gravel, some- 

 times bound together into a solid rock, are found in great abundance 

 around the confines of the lacustrine basin, containing, in different places, 

 pebbles of all the ancient rocks of the adjoining elevated country ; namely, 

 granite, gneiss, mica-schist, clay-slate, porphyry, and others, but without 

 any intermixture of basaltic or other tertiary volcanic rocks. These strata 

 do not form one continuous band around the margin of the basin, being 

 rather disposed like the independent deltas which grow at the mouths of 

 torrents along the borders of existing lakes. 



At Chamalieres, near Clermont, we have an example of one of these 

 deltas, or littoral deposits, of local extent, where the pebbly beds slope 

 away from the granite, as if they had formed a talus beneath the waters 

 of the lake near the steep shore. A section of* about 50 feet in vertical 

 height has been laid open by a torrent, and the pebbles are seen to con- 

 sist throughout of rounded and angular fragments of granite, quartz, 



* Scrope, Geology of Central France, p. 15. 



