18 EEPOBT 1886. 



of both sides of the Atlantic owe their origin to these great lines of de- 

 position, along with the fact, afterwards more fully insisted on by Rogers, 

 that the portions of the crust which received these masses of clebris^ became 

 thereby weighted down and softened, and were more liable than other 

 parts to lateral crushing.' 



Thus in the later Eozoic and early Palaaozoic 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 summits 

 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 

 we may more especially note the continued subsidence of 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. ^ 



We are indebted to a French physicist, M. Faye,^ for an important 

 suggestion on this subject. It is that the sediment accumulated along 

 the shores of the ocean presented an obstacle to radiation, and conse- 

 quently to cooling of the crust, while the ocean floor, unprotected and 

 unweighted, and constantly bathed with currents of cold water, having 

 great power of convection of heat, would be more rapidly cooled, and so 

 would become thicker and stronger. This suggestion is complementary 

 to the theory of Professor Hall, that the areas of greatest deposit on the 



' The connection of accumulation with subsidence was always a familiar con- 

 sideration 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, Rogers, Hunt, Le Conte, Crosby, &c., have followed up Hall's primary sug- 

 gestion, and in England, Hicks, Fisher, Starkie Gardner, Hull, and others, have 

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

 subjects. 



2 Button in Report of U.S. Geological Survey, 1881. From facts stated in this re- 

 port and in my Arcadian Geology, it is apparent that in the Western States and in the 

 coalfield of Nova Scotia shallow-water deposits have been laid down up to thicknesses 

 of 10,000 to 20,000 feet in connection with continuous subsidence. See also a paper 

 by Ricketts in the Geol. Mag. 1883. It may be well to add here that this doctrine 

 of the subsidence of wide areas being caused by deposition does not justify the con- 

 clusion of certain glacialists that snow and ice have exercised a like power in glacial 

 periods. In truth, as will appear in the sequel, great accumulations of snow and ice 

 require to be preceded by subsidence, and wide continental areas can never be covered 

 with deep snow, while of course ice can cause no addition of weight to submerged 

 areas. 



^ Revue Scientifique, 1886. 



