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Show EOCENE PERIOD, [Ch. XVII. grit, sometimes granitiform, reddish and mottled sands ~nd cong1 o mera t es, a1 1 evidently derived fro.m the degradatiOn o f gram' t1' c r ocl<s , and resembling ex.c eedmgly, t. he arena.c eou.s group of the Limagne before dcscnbcd. '1 h1s formatwn IS almost confined to the borders of the basin, and was evidently a littoral deposit. The other member of the formation, the marls, are more or less calcareous, and are associated with limestone and gypsum, which last is worked for agricultural uses, and exactly resembles that of Paris. The analoo·y in the mineral character of the Velay and Paris 0 • basins is rendered more complete by the presence m both of silex in reaular beds. In the limestone I found gyrogonites, 0 • or seeds of the Chara, of the same species as those most com-mon in the Paris basin; and M. Bertrand de Doue has discovered the bones of several mammiferous animals of the same genera as those which characte~ize the ~asins of Auvergne a.nd Paris *. The shells also of this formatiOn correspond specifi· cally with those of Eocene formations in other parts of F~ance. The sand and conglomerate of the fresh-water basm of V elay is entirely free from volcanic pebbles, agreeing in this. respect with the analogous group of the Limagne; but the fact is the more striking in V elay, because the masses of trachyte, clinkstone, and other igneous rock now ab~un~ing in that country, have an aspect of extremely high antiqmty, and constitute a most prominent feature in the geological structure of the district. Yet the non-intermixture of volcanic products with the lacustrine sediment, is just what we should expect when we have ascertained that the imbedded organic remains of those strata are Eocene; whereas the lavas belong in part, if not entirely, to the Miocene period t. . Cantal.-Near Am·illac, in Cantal, another series of freshwater strata occurs, which resembles, in mineral character and organic I .e ma.m s, those of Auvergne and Velay . alr. eady· ded-scl ·ibed. The leading feature of this group, as dlstmgmshe * Descrip. Geognos. des Env.du Puy en Vel~y, 1823. t See above, p. 219, and below, Chap. Xll', Ch. XVII.] LACUSTRlNJo: n'RATA-CANTAL. 237 from the two former, is the immense abundance of silex associated with the calcareous marls and limestone, which last, like the limestone of Auvergne, constitutes an upper member of the fresh-water series. 'l'he formation of the Cantal may be divided into two groups, the lowest composed of gravel, sand, and c1ay, such as might have been derived from the wearing down and decomposition of the granitic schists of the surrounding country; the upper system consisting of siliceous and calcareous marls, contains subordinately gypsum, silex, and limestone-deposits such as the waters of springs charged with carbonate and sulphate of lime, and with silica, may have produced. Fresh-wafer limestone and flints resembling chalk.-To the English geologist, the most interesting feature in the Cantal is the resemblance of the fresh~water limestone, and its accompanying flint, to our upper chalk, a resemblance which, like that of the red sandstone of Auvergne to our secondary ' new red,' is the more important, as being calculated to put the student upon his guard against too implicit a reliance on lithological characters as tests of the relative ages of rocks. 'Vhen we approach Aurillac from the west, we pass over great heathy plains, where the sterile mica-schist is barely covered with vegetation. Near Ytrac, and between La Capelle and Viscamp, we begin to see the surface strewed over with loose broken flints, some of them black in the interior, but with a white external coating, others stained with tints of yellow and red, and looking precisely like the flint gravel of our chalk districts. When heaps of this gravel have thus announced our approach to a new formation, we arrive at length at the escarpment of the lacustrine beds. At the bottom of the hill we sec strata of clay and sand resting on mica-schist ; and above, in the quarries of Belbet, Leybros, and Bruel, a white limestone, in horizontal strata, the surface of which has been hollowed out into irregular furrows, since filled up with broken flint, marl, and ve!!'etable mould. 'IV e recoanize in these cavities ~ b ' filled with dark mould and flint gravel, an exact counterpart to |