THE SUCCESSION OF DEPOSITS. 49 



with the deep hollows extending outward from the river 

 mouths of the American coasts under the ocean, or 

 perhaps they are like those intervening between the banks 

 cast up by the arctic currents on the present American 

 coast, and like those deep channels of the arctic current 

 in the Atlantic recently explored by Dr. Carpenter. 

 Their arrangements geographically, as well as their 

 geological relations, correspond with such views. 



The former consideration with regard to the great lakes 

 deserves especial notice. Drs. Hunt, Newberry, and 

 Spencer have collected many facts to show that the lake 

 basins are connected with one another and with the sea 

 by deep channels now filled up with drift-deposits. It is 

 therefore certain that much of the erosion of these basins 

 may have occurred before the advent of the glacial period, 

 in the Pliocene age, when the American continent was at 

 a higher level than at present. Dr. Newberry has given 

 in the Eeport in the Geology of Ohio, a large collection 

 of facts ascertained by boring or otherwise, wiiich go far 

 to show that were the old channels cleared of drift and 

 the continent slightly elevated, the great lakes would be 

 drained into each other and into the ocean by the valleys 

 of the Hudson and the Mississippi, without any rock 

 cutting, and if the barrier of the Thousand Islands were 

 then somewhat higher, the St. Lawrence valley might 

 have been cut off from the basin of the great lakes. 

 Spencer has, however, shown, on the evidence of differen- 

 tial elevation, that a portion at least of the drainage of 

 the Pliocene lake country may have found its way down 

 the present course of the St. Lawrence valley. 



The latter cause, namely, the possible eroding action of 

 ocean currents, is one more difficult to estimate, yet 

 5 



