NEGATIVE ELEMENTS OF NORTH AMERICA 401 



the fact of deeper subsidence in both of these basins as evidence of a rela- 

 tively negative character. 



Two negative elements remain — the one the youngest, the other the 

 oldest recognized. The Mississippi embayment was a region of sub- 

 sidence during the Tertiary and Cretaceous. Its behavior during earlier 

 periods is indeterminate, but the migration of faunas between North and 

 South America appears to require such a waterway along a favoring coast- 

 line as would be provided by a connection southwestward via the Missis- 

 sippi embayment. We can never know positively, but it is a reasonable 

 presumption, tliat the region formerly had the character which it now 

 possesses and which causes it to stand low. 



The oldest region of which we are able to decipher the stratigraphic 

 history is that about lake Superior. The thickness of Lower and Middle 

 Huronian sediments is given at 6,000 to 18,000 feet, and that of the 

 Upper Huronian as 14,000 feet.* The sediments of the Keweenawan 

 measure 15,000 feet, and although this may not be the true thickness of 

 the beds they nevertheless correspond to a notable subsidence. The sum 

 of these measures shows that the region was one which sank with refer- 

 ence to adjacent areas (presumably Laurentia), and thus during the early 

 ages of continental history exhibited a negative character. The process of 

 intimate folding to which the strata were subjected welded them to the 

 positive element in such manner that they have since shared its elevatory 

 movements; yet the district may still be affected negatively, if we may 

 regard the syncline in the Keweenawan beds and the existing depression 

 as effects of vertical movement due to isostacy. 



The negative elements of the continent whicli have been described may 

 be briefly enumerated as follows: The northern Appalachian (New York 

 to Virginia), the southern Appalachian (North Carolina, Georgia, Ala- 

 bama, and east Tennessee), the southern Eocky Mountain (comprising 

 the Great basin), the northern Eocky Mountain (located in British 

 Columbia and Idaho), the Great Plains (best developed in Indian Terri- 

 tory and Oklahoma, but probably crntinued northward to the northern 

 Eocky Mountain trough), the Illinois and Michigan, the Mississippi, 

 and the Lake Superior negative elements. 



Theoretical Considerations 



The masses which have been distinguished as positive and negative ele- 

 ments appear to stand in contrast according to the predominance of un- 

 conformities or deposits. I regard them as distinct independently of any 



* Chamberlin and Salisbury : Manual of Geology, 1st edition, vol. 2, pp. 176 and 184. 



