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 clialk formation now going on, 

 as is shown by the investigations of Carpenter and 

 VV. 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 



