292 



PHYSIOGRAPHY. 



[chap. 



may have been formed in just the same way as the Globi- 

 gerina ooze, which is now being deposited ; and the pro- 

 portion of shells which have been broken down into mere 

 dust, may have been increased by the pressure to which it 

 has been subjected ; while, ia some parts, the original 

 structure may have been often more or less completely 

 obliterated by the percolation of water; just as in coral- 

 rock, the shapes of the component corals become lost, 

 and in diatomaceous deposits (Chap. XIV.) the individual 

 diatoms run into a sort of opal, from the same cause. 



Fig. 88. — Microscopic section of chalk 

 from Sussex. Magnified about 

 220 diameters. 



Fig. 89. — Atlantic ooze from a depth 

 of 2,250 fathoms. Magnified 

 about 220 diameters. 



With regard to the second difference, it may be remarked, 

 that there is no reason for doubting that the ocean, under 

 •which the chalk was deposited, contained as great an abun- 

 dance' of organisms with siliceous coats and skeletons, as 

 the present Atlantic. The conclusion is, therefore, that 

 such siliceous remains liave once existed in the chalk, but 

 have been dissolved ; and it is supported by the fact, that, 

 even the sponges, the remains of which are found in the 

 chalk in great abundance, have lost the siliceous spicula 



