THE OCEAN. 673 



but a short distance the river-valleys of a coast, because they are ex- 

 cluded by the out-rlowing stream. The bottom of the Hudson is 

 below the sea-level, for a long distance beyond the limit to which the 

 pure ocean-water extends : the same is true of the St. Lawrence, and 

 of many other rivers along the coast. During a progressing sub- 

 mergence, therefore, the ocean would have no power of excavating 

 narrow valleys, unless they happened to be open at both ends, so as to 

 allow the oceanic currents to sweep through. 



As the submergence progressed, there would be, through wave- 

 action, extensive degradation of the ridges and mountains over the 

 surface, and a distribution of the detritus through the intervening de- 

 pressions. In a subsequent emergence of the land, the mountains and 

 ridges would be still further degraded, and the valleys filled by their 

 debris. The laws of sea-coast action would again come into play, and 

 the wear of all new headlands, and the filling of bays, continue to be 

 the result, so long as the emergence was in progress. 



3. Formation of marine deposits, when a continent is mostly without 

 mountain-ranges and valleys. 



If the continent were to a large extent without mountains, the 

 broad flat surface might then lie slightly above or below tide-level 

 at once, or nearly simultaneously, so that, under a small change of 

 level, the waves could sweep across the whole area. It has been 

 shown that the Appalachian Mountains were not raised until after the 

 Carboniferous age, and the greater part of the Rocky Mountains not. 

 before the close of the Cretaceous period. The North American con- 

 tinent was, therefore, in early time, in the condition here supposed ; 

 and the older formations have a corresponding extent and character. 

 The tidal and oceanic currents were almost the only transporters of 

 detritus ; and these agents worked, in one place or another, according 

 to all those various methods which have been above described. There 

 were continental oscillations, causing slight emergences of large areas 

 to alternate with varying submergences ; and, through such changes, 

 the variations in the formations were produced, differences of depths 

 and differences of currents causing transitions from arenaceous to 

 argillaceous or to pebbly accumulations; and the differences required 

 for such changes were so small that the probability of finding the 

 cotemporaneous fragmental deposits of Europe and America, or even 

 of distant parts of one continent, alike arenaceous, argillaceous, or 

 pebbly, is exceedingly small. 



The ocean, like fresh-water streams, has been greatly aided in its 

 geological work by slow chemical change, going on over the surfaces 

 of exposed rocks, often causing them to crumble slowly, or to peal 

 43 



