266 ROLLIN D. SALISBURY 



tion. There is some reason for thinking that the important topo- 

 graphic features of the central Mississippi basin are chiefly of late 

 Tertiary and post-Tertiary origin, developed from a late Tertiary 

 peneplain now represented by the summits of the higher hills and 

 uplands of the region, a few hundred feet above the general level in 

 which the present valleys are sunk. It is true that these summits 

 have sometimes been interpreted as remnants of a Cretaceous pene- 

 plain; but this conclusion is not firmly established, and the alternative 

 suggested above is entitled to consideration, 



3. In the west, the relative uplift in the closing stages of the 

 Tertiary and early Pleistocene was greater. The estimates of the 

 late Tertiary and post-Tertiary uplift here at one point and another 

 range from several hundred feet to several thousand feet. The fig- 

 ures are most definite and perhaps most satisfactory near the Pacific 

 coast. In southern California, the uphft at this time has been 

 estimated at 1,500 feet; in northern Cahfornia, 1,500 to 2,000 feet; 

 and in the Sierras at 3,000 to 6,000 feet. In Oregon, Pleistocene 

 marine fossils are found up to elevations of 1,500 feet, while in and 

 about the Cascade Mountains of Washington, an elevation of several 

 thousand feet, maximum, seems to be well established. 



In British Columbia, the relative upwarp of the corresponding 

 time has been thought to reach an amount comparable to that of the 

 Cascade Range, while, farther north, most of the estimates point to 

 less extensive changes. The old peneplain which is now at an eleva- 

 tion of 6,000 to 9,000 feet in Washington and British Columbia, is 

 thought to descend to 4,000 or 5,000 feet farther north. While 

 the age of the deformation which brought the former peneplain of 

 these northern lands to its present position has not been fixed with 

 precision, the best opinion seems to place it, or at least its initiation, 

 in the late Pliocene and early Pleistocene. 



Students of the western interior have reached no general agree- 

 ment as to the amount of late Tertiary and Quaternary change of 

 level, but there is general agreement that the land of that region was 

 notably higher at the close of the Tertiary, and later, than it had been 

 before. The increase in the height of the land amounted, perhaps, to 

 a few thousand feet in some places, but was probably far from 

 uniform. 



