THE SUCCESSION OF STRATIFIED ROCKS 



incredible they seem, really exist, and are not 

 mere imagination or fancy. 



I want now to refer to these large diagrams 

 (Figs. 41 and 42). Fig. 41 should be carefully 

 examined. It represents what has been dis- 

 covered with regard to the succession of de- 

 posits, those stratified deposits of which I spoke 

 in my last lecture. On the left-hand side is 

 stated the thickness of each deposit, so far as 

 it has been ascertained. 



Most of the extinct animals, all the great 

 extinct animals I have to speak about, come 

 within the upper part. We have an enormous 

 thickness of stratified rock beneath, which 

 contains only marine things, fishes, a few 

 crustaceans, and things of that kind. But all 

 the more interesting great animals have left 

 their bones in the higher strata. The upper- 

 most layer (the recent and Pleistocene) is only 

 some 200 feet in thickness, yet it indicates a 

 period of something like 500,000 years. This 

 being so, you can judge by the thickness of 

 subjacent deposits what an immense lapse of 

 time is represented. Before we get to the 

 chalk we get down nearly 3,000 feet. The 

 thickness of the chalk itself is another 2,500 feet. 



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