666 DYNAMICAL GEOLOGY. 



shore. On existing sea-shores the action in progress, instead of 

 tending to excavate valleys, produces just the contrary effect. It is 

 everywhere wearing off exposed headlands, and filling up bays. 

 The salt waters, in fact, enter but a short distance the river-valleys 

 of a coast, because they are excluded by the out-flowing 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 submergence, 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 cur- 

 rents 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 

 depressions. In a subsequent emergence of the land, the moun- 

 tains 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, as long as the emergence was in 

 progress. 



3. Effects as to the 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 Eocky Moun- 

 tains not before the close of the Cretaceous period. The North 

 American continent was, therefore, in early time, in the condition 

 here supposed ; and the older formations have a corresponding 

 extent and character. There were continental oscillations, causing 

 slight emergences of large areas to alternate with varying sub- 

 mergences, and through such changes the variations in the forma- 

 tions were produced, differences of depths causing transitions from 

 arenaceous to argillaceous or to pebbly and conglomeritic accumu- 

 lations ; and the differences required for such changes are so small 

 that the probability of finding the cotemporaneous fragmental depo- 

 sits of Europe and America, or even of distant parts of one con- 

 tinent, alike arenaceous, argillaceous, or conglomeritic, is exceed- 

 ingly small. The details of the history as regards North America 

 have already been given, and need not be here repeated. 



