ix.J ON A PIECE OF CHALK. 163 



It may be worth while briefly to consider a few of these 

 collateral proofs that the chalk was deposited at the bottom of 

 the sea. 



The great mass of the chalk is composed as we have seen, of 

 the skeletons of Globigerincc, and other simple organisms, 

 imbedded in granular matter. Here and there, however, this 

 hardened mud of the ancient sea reveals the remains of higher 

 animals which have lived and died, and left their hard parts in 

 the mud, just as the oysters die and leave their shells behind 

 them, in the mud of the present seas. 



There are, at the present day, certain groups of animals which 

 are never found in fresh waters, being unable to live anywhere 

 but in the sea. Such are the corals ; those corallines which are 

 called Polyzoa; those creatures which fabricate the lamp-shells, 

 and are called BracJiiopoda ; the pearly Nautilus, and all 

 animals allied to it ; and all the forms of sea-urchins and 

 star-fishes. 



Not only are all these creatures confined to salt water at the 

 present day; but, so far as our records of the past go, the 

 conditions of their existence have been the same : hence, their 

 occurrence in any deposit is as strong evidence as can be obtained, 

 that that deposit was formed in the sea. Now the remains of 

 animals of all the kinds which have been enumerated, occur in 

 the chalk, in greater or less abundance ; while not one of those 

 forms of shell-fish which are characteristic of fresh water has 

 yet been observed in it. 



When we consider that the remains of more than three 

 thousand distinct species of aquatic animals have been dis 

 covered among the fossils of the chalk, that the great majority 

 of them are of such forms as are now met with only in the sea, 

 and that there is no reason to believe that any one of them 

 inhabited fresh water the collateral evidence that the chalk 

 represents an ancient sea-bottom acquires as great force as the 

 proof derived from the nature of the chalk itself. I think you 

 will now allow that I did not overstate my case when I asserted 

 that we have as strong grounds for believing that all the 



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