344 CRETACEOUS FORMATIONS. 



the few quarries of Tilgate grit ; so that these creatures 

 were by no means rare at the period of their existence.* 



The uppermost of these secondary formations is the 

 cretaceous or chalk group, which spreads over a large 

 portion of south-eastern England, and is met with in all 

 parts of Europe. This chalk, which is a carbonate of 

 lime, appears to have been slowly precipitated from 

 tranquil water, as, according to Sir Henry De la Beche, 

 organic remains are beautifully preserved in it. Sub- 

 stances of no greater solidity than common sponges 

 retain their forms, delicate shells remain unbroken, fish 

 even are frequently not flattened, and altogether we 

 have the appearances which justify us in concluding 

 that, since these organic exuviae were entombed, they 

 have been protected from pressure by the consolidation 

 of the rock around them.f 



Beneath the chalk exists what has been called, from 

 its colour derived from a silicate of the protoxide of 

 iron, green sand, and was, no doubt, formed by depo- 

 sition from the same water in which the carbonate of 

 lime was suspended, the green sand falling to the 

 bottom more readily from its greater specific gravity. 

 " The tranquillity," observes Sir Henry De la Beche, 

 "which seems to have prevailed during this great 

 accumulation of siliceo-calcareous matter, whether it 

 may have been a deposit from water, in which it was 

 mechanically suspended, partly the work of living 

 creatures, or in a great measure chemical, is very 

 remarkable. "J 



In the chalk, the remains of the leaves of dicotyle- 

 donous plants and fragments of wood are found more 

 abundantly than in the earlier strata, many of which 



* Dr. Montell, Wonders of Geology. Geology of the South-east 

 of England. 



f Geological Researches; Geological Manual; by Sir Henry 

 Thos. De la Beche, C.B., &c. 



J Ibid. 



