238 
EOCENE PERIOD. 
[Ch. xvir. 
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 referable 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. 
