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Show 226 EOCENE PERIOD• [Ch. XVII. Auver ne -We allude to the lacustrine basins of Au~ergne, Cantal, gan d· Velay, h 't f which may be seen m the t e SI e o No !'ln. 0 47 ········~···· .. ···········- 0 J6 Montlucon 0 ~· It; • -~". . ..... /i?A:.. I. ............ . ~ c) . ~ ~ ~ ~ to k::::;:;\1 Vulcanic Ch. XVII.] LACUSTRINE STRATA-AUVERGN:E. 227 annexed Map*· They appear to be the monuments of ancient lakes which may have resembled in geographical distribution some of those now existing in Switzerland, and may like them have occupied the depressions in a mountainous country, and have been each fed by one or more rivers and torrents. The country where they occur is almost entirely composed of granite, and different varieties of granitic schist, with here and there a few patches of secondary strata much dislocated, and which have probably suffered great denudation. There are also some vast piles of volcanic rock, (see the Map,) the greater part of which are newer than the fresh-water strata, often resting upon them, whilst a small part were evidently of contemporaneous ongm. Of these igneous rocks we shall treat more particularly in the nineteenth chapter, and shall :first turn our attention exclusively to the lacustrine beds. The most northern of ~he fresh-water 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 Limagne d'Auvergne. It is inclosed by two parallel primitive 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 latter river from the Sioule, on the west t. 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, limestone, and some subordinate groups, none of which observe a fixed and invariable order of superposition. The ancient borders of the lake, wherein the fresh-water strata were accumulated, may generally be traced with precision, the granite and other ancient rocks rising up boldly from the level country. The precise junction, however, of the lacustrine and granitic beds is rarely seen, as a small valley usually intervenes between them. The fresh- * The following account of the fresh-water formations of Central France is the result of observations made in the summer of 1828, in company with Ml·. Mur. chison. t Scrope, Geology of Central France, p. 15. Q2 |