GEOLOGY OF THE ATLANTIC OCEAN. 49 



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 comple- 

 mentary to the theory of Professor Hall, that the areas of greatest 

 deposit on the margins of the ocean are necessarily those of greatest 

 folding and consequent elevation. We have thus a hard, thick, resist- 

 ing ocean-bottom which, as it settles down toward the interior, under 

 the influence of gravity, squeezes upward and folds and plicates all the 

 soft sediments deposited on its edges. The Atlantic area is almost an 

 unbroken cake of this kind. The Pacific area has cracked in many 

 places, allowing the interior fluid matter to ooze out in volcanic ejec- 

 tions. It may be said that all this supposes a permanent continuance 

 of the ocean-basins, whereas many geologists postulate a mid- Atlantic 

 continent to give the thick masses of detritus found in the older forma- 

 tions both in Eastern America and Western Europe, and which thin 

 off in proceeding into the interior of both continents. I prefer, with 

 Hall, to consider these belts of sediments as in the main the deposits 

 of northern currents, and derived from Arctic land, and that, like the 

 great banks of the American coast at the present day, which are being 

 built up by the present Arctic current, they had little to do with any 

 direct drainage from the adjacent shore. We need not deny, however, 

 that such ridges of land as existed along the Atlantic margins were 

 contributing their quota of river-borne material, just as on a still greater 

 scale the Amazon and Mississippi are doing now, and this especially on 

 the sides toward the present continental plateaus, though the greater 

 part must have been derived from the wide tracts of Laurentian land 

 within the Arctic Circle or near to it. 



It is further obvious that the ordinary reasoning respecting the ne- 

 cessity of continental areas in the present ocean-basins would actually 

 oblige us to suppose that the whole of the oceans and continents had 

 repeatedly changed places. This consideration opposes enormous 

 physical difficulties to any theory of alternations of the oceanic and 

 continental areas, except locally at their margins. I would, however, 

 refer you for a more full discussion of these points to the address to 

 be delivered to-morrow by the President of the Geological Section. 

 But the permanence of the Atlantic depression does not exclude the 

 idea of successive submergences of the continental plateaus and mar- 

 ginal slopes, alternating with periods of elevation, when the ocean 

 retreated from the continents and contracted its limits. In this re- 

 spect the Atlantic of to-day is much smaller than it was in those times 

 when it spread widely over the continental plains and slopes, and much 

 larger than it has been in times of continental elevation. This leads 



