116 Proceedivffs, 



iu a soluble state and has been again re-solidified. Some- 

 times on breaking open a flint nodule we find the cast of the 

 sponge itself in flint. I have a small specimen here which 

 looks like a minute petrified bath-sponge, the sarcode of 

 which seems to have been replaced by flint, and this, I 

 believe, is an example of the state in which the sponges are 

 usually found. We do not know what the black colour of 

 flint is due to ; it has been ascribed to iron, manganese, and 

 even organic matter. That the flint was formed during the 

 continuance of the Cretaceous epoch, and is not always a sub- 

 sequent production, is certain, because we find in Belgium 

 evidence that some of the chalk was upraised and abraded off, 

 the flint nodules it contained being woi'n into pebbles, and 

 mixed with these pebbles are unworn fossils of the chalk. 



The genera of cephalopods I have already desci'ibed ; the 

 Nautilus, Ammonite, and Belemnite lived in the chalk ocean, 

 but the decreasing proportion in which they stand to other 

 molluscs is noticeable. The Ammonites, which attained their 

 maximum number in a prior formation, the Lias, are now 

 dying out, and in connection with this impending extinction 

 a very remarkable thing is to be noted. In no other 

 formation do we get such an extraordinary variation in the 

 forms of Ammonites as in the Cretaceous. It seems just as 

 if Nature said to herself — " These Ammonites seem to be 

 getting crowded out by other auimals ; let me see if I cannot 

 modify their form so that they shall be able to hold their 

 own." So she altered their shells into all sorts of strange 

 shapes, but it was of no use ; they died out and became 

 extinct before the next epoch, the Tertiary, fairly commenced. 

 I have figured these various forms ; they have received diffe- 

 rent generic names, but they are essentially Ammonite shells. 

 The same fate befel the Belemnite, for we do not find that in 

 any formation subsequent to the Chalk. 



I remarked, in treating of the Wealden, that a great change 

 in the vegetable life of this region shortly took place, and 

 this is one of the reasons why we regard the Neocomian as a 

 formation distinct from the Cretaceous ; for we find that up 

 to the top beds of the former all the forms of vegetation were 



