74 THE HISTORY OF THE NORTH ATLANTIC 



interior of both continents. I prefer, as already stated, to 

 consider these belts of sediment as the deposits of north- 

 ern currents, and derived from arctic land, and that, like the 

 great banks off 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 Lauren- 

 tian land within the Arctic Circle, or near to it. It is further 

 obvious that the ordinary reasoning respecting the necessity of 

 continental areas in the present ocean basins would actually 

 oblige us to suppose that the whole of the oceans and conti- 

 nents had repeatedly changed places. This consideration op- 

 poses enormous physical difficulties to any theory of alterna- 

 tions of the oceanic and continental areas, except locally at their 

 margins. 



But the permanence of the Atlantic depression does not ex- 

 clude the idea of successive submergences of the continental 

 plateaus and marginal slopes, alternating with periods of eleva- 

 tion, when the ocean retreated from the continents and con- 

 tracted its limits. In this respect 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 us to 

 the further consideration that, while the ocean beds have been 

 sinking, other areas have been better supported, and constitute 

 the continental plateaus j and that it has been at or near the 

 junctions of these sinking and rising areas that the thickest de- 

 posits of detritus, the most extensive foldings, and the greatest 

 ejections of volcanic matter have occurred. There has thus 



