58 ON A PIECE OF CHALK 



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 Polycoa ; those 

 creatures which fabricate the lamp-shells, and are 

 called Brachiopoda ; the pearly Nautilus, and all ani- 

 mals 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 discovered 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 rea- 

 son to believe that any one of them inhabited fresh 

 water the collateral evidence that the chalk repre- 

 sents an ancient sea-bottom acquires as great force 

 as the proof derived from the nature of the chalk it- 

 self. I think you will now allow that I did not over- 



