OHIGIN OF THE SUBMARINE VALLEYS 221 



12,000 or 15,000 feet, it may seem difficult to explain these lower reaches 

 by the h3^pothesis of atmospheric action during a period of emergence 

 on account of the stupendous changes of level of land and sea required ; 

 yet the writer has ventured to adopt this hypothesis, in which he has 

 been confirmed by many years of research. But from the broad stand- 

 point the complex conditions doubtless qualify the simple hypothesis of 

 the former elevation of the land with its consequent sculpturing by at- 

 mospheric agents. While some of the valleys may be attributed to 

 tectonic ororogenic, or occasional ones to volcanic causes, no explanation 

 based on these causes has been worked out in detail: consequently the 

 author has been led, after presenting the facts given in this and other 

 papers, to emphasize particularly the resemblances between these sub- 

 marine valleys and land features, with the conclusion that the former 

 were sculptured on the great continental slopes by atmospheric agents, 

 which implies a greater change of level of land and sea than the 2,000 

 or 3,000 feet above mentioned. If it were a question of simple elevation, 

 it would amount to 12,000 or 15,000 feet higher than at present along the 

 border of the continent. This great elevation, however, may have been 

 much reduced by an unequal bending down of the continental slope, or 

 indeed to some extent by a shifting of the oceanic waters. Then also 

 arises the question. What became of the waters, and also what were the 

 causes of these great continental movements, about which we know 

 nothing ? The problem thus becomes so complex that the writer has to 

 confine himself to the study of the resemblances above mentioned. 

 While the features along the Atlantic coast are repeated on the eastern 

 side of that basin and elsewhere, the author could not possibly imply a 

 general drainage of the basins, but rather that there have been alterna- 

 tions, whereby great regions have been elevated while others have been 

 depressed, as, for instance, the West Indian islands alternating in alti- 

 tude with the lands of Central America. We know that in the epeiro- 

 genic movements the changes of level are unequal, with the rate of ele- 

 vation or subsidence increasing or diminishing, and from the writer's 

 observations such rates increase on approaching mountain regions and 

 diminish in the direction of the plains. This, extended to the great 

 continental slopes, would favor the theory of their having been abnor- 

 mally bent downward ; consequently the land may not necessarily have 

 stood 12,000 or 15,000 feet higher than now, although the bottom of the 

 slopes had been emerged to that extent. Still the land stood very much 

 higher than at present, probably sufficient to give rise to glacial condi- 

 tions in the north. 



While the amphitheaters, coves, or canyons indenting the edge of the 

 submerged continental shelf are known to have a breadth increasing 



