74 EXPEDITION OF THE " CHALLENGER " m 



a fossil state, constitute extensive rocks of tertiary 

 age at Caltanisetta, Zante, and Oran, on the 

 shores of the Mediterranean. 



Moreover, in the fresh-water rotten-stone beds 

 of Bilin, Ehrenberg had traced out the metamor- 

 phosis, effected apparently by the action of perco- 

 lating water, of the primitively loose and friable 

 deposit of organized particles, in which the silex 

 exists in the hydrated or soluble condition. The 

 silex, in fact, undergoes solution and slow redepo- 

 sition, until, in ultimate result, the excessively 

 fine-grained sand, each particle of which is a 

 skeleton, becomes converted into a dense opaline 

 stone, with only here and there an indication of an 

 organism. 



From the consideration of these facts, Ehren- 

 berg, as early as the year 1839, had arrived at the 

 conclusion that rocks, altogether similar to those 

 which constitute a large part of the crust of the 

 earth, must be forming, at the present day, at the 

 bottom of the sea ; and he threw out the sugges- 

 tion that even where no trace of organic structure 

 is to be found in the older rocks, it may have been 

 lost by metamorphosis. 1 



1 Uebcr die noch jetzt zaTilrcich lebende Thierarten der Krcide- 

 bildung und den Organismus dcr Polythalamien. Abhandlungen 

 der Kon. Akad. der Wisstnchaften. 1839. Berlin. 1841. I am 

 afraid that this remarkable paper has been somewhat overlooked 

 in the recent discussions of the relation of ancient rocks to 

 modern deposits. 



