42 



to the sea, and it is therefore a marine deposit. The 

 chalk is in this neighbourhood from 300 to 400 feet thicij. 

 Now, where the sea was two miles or more in depth, it 

 would be too far from land to receive these deposits from 

 a shore; but geologists explain the great thickness of some 

 of these beds by the sripposition that the floor of that sea 

 was continually sinking, and receiving deposits from the 

 animals which infested it. In the chalk we frequently find 

 the perfect remains of animals, such as fishes, &c. ; but 

 ■we meet with no bones of land animals, nor any plants ex- 

 cept sea-weeds, and here and there a piece of drift wood ; 

 and we therefore conclude that the white chalk is the pro- 

 duct of an open sea of considerable depth. The existence 

 of turtles and saurians, tho ichthyosaurus or 6sh-lizard, 

 found by me in chalk at Folkestone and by Mr. Carter, 

 at Cambridge, the coniosaurus in my collection from Charing, 

 and the pterodctyl or winged lizard in the chalk near 

 Maidstone, implies, no doubt, a neighbouring land, which 

 some geologists' believe to have been on the west 

 and south. I have been asked to explain the origin 

 of the layers of flint in chalk, whether in continuous 

 sheets, or tabular veins, or in the form of nodules. 

 This is more difficult to explain than is the origin 

 of white chalk. Sir Charles Lyell tells us that " no such 

 siliceous masses are as yet known to accompany the aggre- 

 gation of chalky mud in modern coral-reefs." The flint 

 abounds mostly in the uppermost chalk, and becomes more 

 rare or is entirely wanting as we descend ; but this rule 

 does not hold universally throughout Europe. Some por- 

 tion of the flint may have oeea derived from the decom- 

 position of sponges and other zoophytes provided with 

 siliceous skeletons ; for it is a fdct that siliceous spicuhe, or 

 the minute bones of sponges, are often met with in flinty 

 nodules, and may have served at least as points of attraction 

 to some of the siliceous matter when it was in the act of 

 separating from chalky mud during the process of solidifi- 

 cation. But there are other copious sources, of which the 

 decomposition of felspar is one, and the disintegration of 



