CHAP. XXXIV.] NORTHERN DRIFT. 199 



drained by the head waters of the Mississippi, Missouri, and their 

 northern tributaries.* For this and other reasons, into which I 

 can not 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, were nevertheless anterior in date to the loam of 

 Natchez and Vicksburg. 



There exist in Canada, in the Niagara district, in New York, 

 and other states north of the Ohio, lacustrine and swamp deposits 

 of marl and bog-earth, including the bones of extinct quadrupeds, 

 such as the mastodon, elephant, castoroides, and others, associated 

 with land and fresh-water shells of recent species, which are 

 decidedly post-glacial, and often found in hollows in the drift. 

 These may be of contemporaneous date with the loam of Port 

 Hudson and Natchez. 



The northern drift, however, is by no means all of the same 

 age, and as the period of glaciers and icebergs freighted with 

 erratics is still going on, and has now a wide range in the tem 

 perate parts of the Atlantic, bordering the eastern shores of North 

 America, so must we naturally suppose that certain parts of the 

 drift, especially those found at lower levels, and near the sea, 

 may not be more ancient than the loam of the western bluffs of 

 the Mississippi. 



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



