238 EOCENE PERIOD. [Ch. XVII. 



the appearances so frequently presented on the furrowed sur- 

 face of our white chalk. Proceeding onwards from these 

 quarries, along a road made of the white limestone, which re- 

 flects as glaring a light in the sun, as do our roads composed of 

 chalk, we reach, at length, -in the neighbourhood 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 colour, 

 like the exterior of the flinty nodules of our chalk. In these 

 last the hard white substance has been ascertained to consist, in 

 some instances, wholly of siliceous matter, and sometimes to 

 contain a small admixture of carbonate of lime *, and the ana- 

 lysis of those of the Cantal would probably give the same 

 results. The Aurillac flints have precisely the appearance of 

 having separated from their matrix after the siliceous and cal- 

 careous matter had been blended together. The calcareous 

 marl sometimes occupies small sinuous cavities in the flint, 

 and the siliceous nodule, when detached, is often as irregular 

 in form as those found in our chalk. 



By what means, then, can the geologist at once decide 

 that the limestone and silex of Aurillac are referrible to an 

 epoch entirely distinct from that of the English chalk ? It is 

 not by reference to position, for we can merely say of the 

 lacustrine beds, as we should have been able to declare of the 

 true chalk had it been present, that they overlie the granitic 

 rocks of this part of France. It is by reference to the organic 

 remains that we are able to pronounce the formation to belong 

 to the Eocene tertiary period. Instead of the marine Alcyonia 

 of our cretaceous system, the silicified seed-vessels of the Chara, 

 a plant which grows at the bottom of lakes, abound in the 

 flints of Aurillac, both in those which are in situ and those 

 forming the gravel. Instead of the Echinus and marine tes- 

 tacea of the chalk, we find in the marls and limestones the 

 shells of the Planorbis, and other lacustrine testacea, all of 



* Phillips, Geol Trans. First Series, vol. v. p. 22. Outlines of Geology, 

 p. 95. 



