Ch. XVII.] 



LACUSTRINE STRATA CANTAL. 237 



from the two former, is the immense abundance of silex asso- 

 ciated with the calcareous marls and limestone, which last, like 

 the limestone of Auvergne, constitutes an upper member of the 

 fresh-water series. 



The formation of the Cantal may be divided into two 

 groups, the lowest composed of gravel, sand, and clay, such as 

 might have been derived from the wearing down and decom- 

 position 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-water 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 ac- 

 companying flint, to our upper chalk, a resemblance which, 

 like that of the red sandstone of Auvergne to our secondary 

 ' new red, 7 is the more important, as being calculated to 

 put the student upon his guard against too implicit a reli- 

 ance on lithological characters as tests of the relative ages of 

 rocks. 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, 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 dis- 

 tricts. When heaps of this gravel have thus announced our 

 approach to a new formation, we arrive at length at the escarp- 

 ment of the lacustrine beds. At the bottom of the hill 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 furrows, since filled up with broken flint, 

 marl, and vegetable mould. We recognize in these cavities, 

 filled with dark mould and flint gravel, an exact counterpart to 



