Ill EXPEDITION OF THE "CHALLENGER" 107 



place. But the formation of greensand, and still 

 more that of the "red clay" (if the Challenger 

 hypothesis be correct) affords an insight into a 

 new kind of metamorphosis not igneous, but 

 .aqueous by which the primitive nature of a 

 deposit may be masked as completely as it can be 

 by the agency of heat. And, as Wyville Thomson 

 suggests, in the passage I have quoted above (p. 

 17), it further enables us to assign a new cause 

 for the occurrence, so puzzling hitherto, of 

 thousands of feet of unfossiliferous fine-grained 

 schists and slates, in the midst of formations 

 deposited in seas which certainly abounded in life. 

 If the great deposit of " red clay " now forming in 

 the eastern valley of the Atlantic were meta- 

 morphosed into slate and then upheaved, it would 

 constitute an " azoic " rock of enormous extent 

 And yet that rock is now forming in the midst of 

 a sea which swarms with living beings, the great 

 majority of which are provided with calcareous or 

 silicious shells and skeletons ; and, therefore, are 

 such as, up to this time, we should have termed 

 eminently preservable. 



Thus the discoveries made by the Challenger 

 expedition, like all recent advances in our 

 knowledge of the phenomena of biology, or 

 of the changes now being effected in the 

 structure of the surface of the earth, are in 

 accordance with, and lend strong support to, 

 that doctrine of Uniformitarianism, which, fifty 



