64 THE DOCTRINE OF DESCENT. 
ground down into sand, and in other places the strata 
are purely chalky or marly. But the vagueness of the 
limitations of strata in situation, and still more in time, 
may be estimated by the fact that we are fully justi- 
fied in speaking of the chalk formation now going on, 
as is shown by the investigations of Carpenter and 
W. Thompson on the constitution of the deep sea- 
bottom of the Atlantic. To the early chalk period 
belongs a great fresh-water deposit, and likewise the 
Wealden, a formation of peat and bog occasioned by 
upheavals, which contains a number of remains of fresh- 
water and terrestrial animals, besides a peculiar sort of 
coal, 
The oolitic strata appear more definite, mostly lying 
regularly over each other in distinct deposits, more rarely, 
as in the Alps, raised up by later dislocations. The rocks 
themselves, betray that the depositions took place in 
wide seas, for the most part calm or deep, and this is 
rendered a certainty by the scanty vegetal remains and 
the far more abundant animal remains which they con- 
tain. In the apparently very sharp limitation of the 
oolitic formation, both above and below, the older geo- 
logy found a main prop for the assertion, that compara- 
tively quiet periods of long duration alternated with 
catastrophes destroying and re-creating everything. To 
avoid any misapprehension we must, however, add that 
the oolitic period already possessed vast and highly 
integrated continents, as it will likewise be seen that 
during this era the higher terrestrial animals made 
their appearance. 
The characters shown by the three great divisions 
of the triassic formation are very various, especially as 
