Presidential Address. 221 



bonic acid, then existing in the atmosphere perhaps more 

 abundantly than at present, under whose influence the 

 hardest of the gneissic rocks gradually decay. The Arctic 

 lands were subjected in addition to the powerful mechanical 

 force of frost and thaw. Thus every shower of rain and 

 every swollen stream would carry into the sea the products 

 of the waste of land, sorting them into fine clays and coar- 

 ser sands ; and the cold currents which cling to the ocean 

 bottom, now determined in their courses, not merely by the 

 earth's rotation, but also by the lines of folding on both 

 sides of the Atlantic, would carry south-westward, and pile 

 up in marginal banks of great thickness, the debris produced 

 from the rapid waste of the land already existing in the 

 Arctic regions. The Atlantic, opening widely to the north, 

 and having large rivers pouring into it, was, especially, the 

 ocean characterised, as time advanced, by the prevalence 

 of these phenomena. Thus throughout the geological history 

 it has happened that, while the middle of the Atlantic has 

 received merely organic deposits of shells of Foraminifera 

 and similar organisms, and this probably only to a small 

 amount, its margins have had piled upon them beds of de- 

 tritus of immense thickness. Professor Hall, of Albany, 

 was the first geologist who pointed out the vast cosmic im- 

 portance of these deposits, and that the mountains of both, 

 sides of the Atlantic owe their origin to these great lines of 

 deposition, along with the fact, afterwards more fully in- 

 sisted 'on by Eogers, that the portions of the crust which 

 received these masses of debris became thereby weighted 

 down and softened, and were more liable than other parts 

 to lateral crushing. ] 



1 The connection of accumulation with subsidence was always a 

 familiar consideration with geologists ; but Hall seems to have 

 been the first to state its true significance as a geological factor, 

 and to see that those portions of the crust, which are weighted 

 down by great detrital accumulations, are necessarily those which, 

 in succeeding movements, were elevated into mountains. Other 

 American geologists, as Dana, Eogers, Hunt, Le Conte, Crosby, &c, 

 have followed up Hall's primary suggestion, and in England, Hicks, 

 Fisher, Starkie Gardiner, Hull, and others, have brought it under 

 notice, and it enters into the great generalisations of Lyell on these 

 subjects. 



