204 UPPER EOCENE STRATA— CANTAL. [Cii. XV. 



Loire, near tlie town of Le Puy, in Velay ; and another occurs nea^ 

 Aurillac, in Cantal. The leading feature of the formation last mentioned, 

 as distinguished from those of Auvergne and Velay, is the immense 

 abundance of silex associated with calcareous marls and limestone. 



The whole series may be separated into two divisions ; the lower, com- 

 posed of gravel, sand, and clay, 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 calca- 

 reous marls, contains subordinately gypsum, silex, and limestone. 



The resemblance of the freshwater limestone of the Cantal, and its 

 accompanying flint, to the upper chalk of England, is very instructive, 

 and well calculated to put the student upon his guard against rely- 

 ing too implicitly on mineral character alone as a safe criterion of rela- 

 tive age. 



When 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, the surface is 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 in appearance 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 which rises before us, we see 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 furi'ows, since filled up with 

 broken flint, marl, and dark vegetable mould. In these cavities we recog; 

 nize an exact counterpart to those which are so numerous on the fur- 

 rowed surface of our own white chalk. Advancing from these quarries 

 along a road made of the white limestone, which reflects as glaring a light 

 in the sun as do our roads composed of chalk, we reach, at length, in 

 the neighborhood of Aurillac, hills of limestone and calcareous marl, in 

 horizontal strata, separated in some places by regular layers of flint in 

 nodules, the coating of each nodule being of an opaque white color, like 

 the exterior of the flinty nodules of our chalk. 



The abundant supply both of siHceous, calcareous, and gypseous mat- 

 ter, which the ancient lakes of France received, may have been connected 

 with the subterranean volcanic agency of whi(;h those regions were so 

 long the theati'e, and which may have impregnated the springs with min- 

 eral matter, even before the great outbreak of lava. It is well known that 

 the hot spiings of Iceland, and many other countries, contain silex in solu- 

 tion ; and it has been lately afflrmed, that steam at a high temperature is 

 capable of dissolving quartzose rocks without the aid of any alkaline or 

 other flux.** Warm water charged with siliceous matter would immedi- 

 ately part with a portion of its silex, if its temperature was lowered by 

 mixing with the cooler waters of a lake. 



* See Proceedings of Royal Soe., 'No, 44, p. 233. 



