264 NORTHERN DRIFT. [CHAP. XXXIV. 



oscillations, would require sixteen thousand years for 

 its accomplishment. But the section at Cincinnati 

 seems to imply two oscillations, and there would pro 

 bably be pauses, and a stationary period, when the 

 downward movement ceased, and was not yet changed 

 into an upward one. Nor ought we to imagine that 

 the whole space was always in motion at once. 



When we have at length done our best to trace 

 back the history of the more modern and more 

 ancient alluvial formations of the Mississippi, the 

 question still remains, what may be their age rela 

 tively to the great body of the drift containing erratic 

 blocks in the northern latitudes of this same con 

 tinent. The terraces of gravel and loam bordering 



o o 



the Ohio, and those on a larger scale, but of the 

 same age, which constitute many of the eastern bluffs 

 of the Mississippi, tire evidently features of sub 

 ordinate importance in the physical configuration of 

 the continent. But to explain the origin of the 

 northern drift of the Canadian lake district, and of 

 the St. Lawrence, as I have endeavoured to show in 

 my former &quot; Travels,&quot; requires a reference to such 

 changes as would imply the submergence of a great 

 part of the continent drained by the head waters of 

 the Mississippi, Missouri, and their northern tribu 

 taries.* For this and other reasons, into which 1 

 cannot now enter, I presume that the great mass of 

 the most elevated drift in the north, and the glacial 

 grooving and polishing of the rocks, although they 

 belong to a very modern era in the earth s history, 



* See vol. i. ch. ii. p. 47. and vol. ii. ch. xix. p. 99. 



