48 THE POPULAR SCIENCE MONTHLY. 



Eozoic tracts of continent were in the earliest times areas of depo- 

 sition, and that the first elevations of land out of the primeval ocean 

 must have differed in important points from all that have succeeded 

 them ; but they were equally amenable to the ordinary laws of denu- 

 dation. Portions of these oldest crystalline rocks, raised out of the 

 protecting water, were now eroded by atmospheric agents, and espe- 

 cially by the carbonic 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 w r ere 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 tine clays and 

 coarser 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 southwestward, 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 character- 

 ized, 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 

 detritus of immense thickness. Professor Hall, of Albany, was the first 

 geologist who pointed out the vast cosmic importance of these depos- 

 its, and that the mountains of both sides of the Atlantic owe their 

 origin to these great lines of deposition ; along with the fact, afterward 

 more fully insisted upon by Rogers, 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. 

 Thus in the later Eozoic and early Palaeozoic times, which succeeded 

 the first foldings of the oldest Laurentian, great ridges were thrown 

 up. along the edges of which were beds of limestone, and on their sum- 

 mits and sides thick masses of ejected igneous rocks. In the bed of 

 the central Atlantic there are no such accumulations. It must have 

 been a flat, or slightly ridged, plate of the ancient gneiss, hard and 

 resisting, though perhaps with a few cracks, through which igneous 

 matter welled up, as in Iceland and the Azores in more modern times. 

 In this condition of things we have causes tending to perpetuate and 

 extend the distinctions of ocean and continent, mountain and plain, 

 already begun ; and of these Ave may more especially note the con- 

 tinued sulisidenee <>(' the areas of greatest marine deposition. This 

 has long attracted attention, and affords very convincing evidence of 

 the connection of sedimentary deposit as a cause with the subsidence 

 of the crust. 



