82 EXPEDITION OF THE " CHALLENGER " in 



of percolating water may convert the originally 

 soft and friable, fine-grained sandstone into a 

 dense, semi-transparent opaline stone, the silicious 

 organized skeletons being dissolved, and the silex 

 re-deposited in an amorphous state. Whether 

 such a metamorphosis as this occurs in submarine 

 deposits, as well as in those formed in fresh water, 

 does not appear; but there seems no reason to 

 doubt that it may. And hence it may not be 

 hazardous to conclude that very ordinary meta- 

 morphic agencies may convert these polar caps into 

 a form of quartzite. 



In the great intermediate zone, occupying some 

 110 of latitude, which separates the circumpolar 

 Arctic and Antarctic areas of silicious deposit, the 

 Diatoms and Radiolaria of the surface water and 

 the sponges of the bottom do not die out, and, so 

 far as some forms are concerned, do not even 

 appear to diminish in total number ; though, on a 

 rough 'estimate, it would appear that the propor- 

 tion of Radiolaria to Diatoms is much greater than 

 in the colder seas. Nevertheless the composition 

 of the deep-sea mud of this intermediate zone is 

 entirely different from that of the circumpolar 

 regions. 



The first exact information respecting the 

 nature of this mud at depths greater than 1,000 

 fathoms was given by Ehrenberg, in the account 

 which he published in the " Monatsberichte " of 



