224 Onth< Surfaa Geology of tfo Basin of the 



lake basin were driven northward and finally altogether dis- 

 solved, we arc not left entirely to conjecture. 



ismical causes possibly and probably had the chief agency 

 in producing this result, but we have unmistakable evidence 

 of at least the co-operation of another and perhaps no less potent 

 cause, viz., continental depression. 



\i' a cosniical cause had simply increased the annual tem- 

 perature till the glaciers were all melted, without the action of 

 any other agent, we should never have had the accumulation 

 of drift deposits which now occupy all the glacial area; but the 

 drainage streams, changed in all their courses from ice to water, 

 would have flowed freely and rapidly away through their deep- 

 ly cut channels to deposit their abundant sediments only where 

 their transporting power was arrested, in the depths o( the 

 ocean. 



Instead of this, we everywhere find evidence that this flow 

 was checked, and a basin of quiet water formed by an advance 

 of tin' ocean consequent upon a subsidence of the land. On the 

 Atlantic and Gulf coasts this depression progressed until the 

 sea-level was more than .'.no tVet higher than now. The effect 

 of this depression was to deeply submerge the eastern margin 



of the continent, and cover it with the " Champlaiu " (days. 



It is evident that at this period the drainage from the great 

 water-shed of the continent musl have been mel by the quiet 

 of the ocean almost at the Bources of the present drain- 

 ing stream-, and as the "dead water" gradually crept up the 

 valleys, arresting the transporting power oi their currents, their 



old channels would be silted up and obliterated, and their val- 



partially filled with materials for their subsequent terra* 

 In the advance ami subsequent recession of the line of "dead 

 water" we have ample cause for all our terrace phenomena. 



This continental depression accounts satisfactorily for the 

 filling of the old channels of the Mississippi and the ( )hio, as a 

 depression of 500 feet would bring the ocean nearly to Pitts- 



pgb on the ( diio. to St. Paul on the Mississippi. 



