190 LYELL'S ELEMENTS OF GEOLOGY. 



Chalk Flints. 



found in the Maestricht limestone, genera which have not yet 

 been detected in strata newer than the chalk. In the same 

 formation, also, large turtles have been found, and a gigantic 

 reptile, the Mosasaurus, or fossil Monitor, some of the vertebrce 

 of which appear also in the English chalk.* The osteological 

 characters of this oviparous quadruped prove it to have been 

 intermediate between the living Monitors and Iguanas ; and, from 

 the size of the head, vertebras, and other bones, it is supposed 

 to have been twenty-four feet in length. 



The existence of such turtles and saurians seems to imply 

 some neighbouring land, on the sandy shores of which these 

 creatures may have laid their eggs. But a few small islets in 

 mid ocean, like Ascension, so much frequented by turtles, may 

 perhaps have afforded the required retreat to these cretaceous 

 reptiles. 



Origin of the Jlint in chalk. It is difficult to give a satis- 

 factory explanation of the origin of the flint in chalk, whether 

 it occurs in nodules or continuous layers. It seems that there 

 was originally siliceous as well as calcareous earth in the muddy 

 bottom of the cretaceous sea, at least when the upper chalk was 

 deposited. Whether both these earths could have been alike 

 supplied by the decay of organic bodies may be matter of spe- 

 culation ; but what was said of the origin of Tripoli (see p. 39.) 

 shows how microscopic infusoria can give rise to dense masses 

 of pure flint. The skeletons of many living sponges consist of 

 needles or spicula of flint, and these are found very abundantly 

 in the flints of the chalk. There are also other living zoophytes, 

 which have the power of secreting siliceous matters from the 

 waters of the sea, just as mollusca secrete calcareous particles. 



From whatever source the mud derived its silex, we may at- 

 tribute the parallel disposition of the flinty layers to successive 

 deposition. The distances between the layers, says Dr. Buck- 

 land, must have been regulated by the intervals of precipitation, 

 each new mass forming at the bottom of the ocean a bed of 

 pulpy fluid, which did not penetrate the preceding bed on which 

 it rested, because the consolidation of this last was so far advanc- 

 ed as to prevent such intermixture, f Nevertheless the separa- 

 tion of the flint into layers, so distinct from the chalk, is a sin- 

 gular phenomenon, and not yet accounted for. Perhaps, as the 

 specific gravity of the siliceous exceeds that of the calcareous 

 particles, the heavier flint may have sunk to the bottom of each 

 stratum of soft mud ? 



* See Mantell's Geol. of S. E. of England. 

 t Geol. Trans., First Series, vol. iv. p. 420. 



