934 HISTORICAL GEOLOGY. 



fracture-planes thousands of feet in displacement in the mountain ranges of 

 the Great Basin, the High Plateaus of Utah, the Wasatch Mountains, and 

 the Sierra Nevada. 



Through a study of the river systems of the Sierra Nevada, it has been 

 proved by LeConte (1886) that a great elevation of the Sierra took place at 

 or near the close of the Pliocene. Tiie drainage of the Sierra is chiefly to 

 the westward, the eastern front being very steep. Whitney describes in his 

 Eeport (1865) the facts respecting an early system of valleys having been 

 covered up and obliterated by basaltic eruptions, and the new and much 

 deeper system of subsequent time (page 300). He illustrates also, by a 

 plate in his work on the Auriferous Gravels (1880), the difference in the 

 depth of erosion of the two systems, the earlier that occupied all Cretaceous 

 and Tertiary time, and the later, of subsequent time after the eruptions. 

 In view of these and related facts, LeConte urges that the deeper erosion 

 by the existing streams, although their time of work was short compared 

 with that of the earlier system which existed through the Cretaceous and 

 Tertiary, proves that a great elevation of the Sierra Nevada, increasing the 

 fluvial denuding power, took place soon after the Pliocene ; and that this 

 was accomplished by a rise along a fault-plane having the course of the 

 steep eastern front of the range. It is to be remarked that the Glacial period 

 followed the Pliocene ; and its glaciers and abundant precipitation would 

 account for part of the profound denudation of the later rivers. But this 

 fact does not invalidate seriously the conclusions. It is sustained through 

 additional facts by other geologists, including Lindgreu and Diller. 



The eastern border of the continent underwent only small changes. At 

 the close of the Eocene some modification of the surface occurred within the 

 Mexican Gulf which put an end to the deposition of true marine beds along 

 its northern beds west of Florida. The only Miocene beds recognized are of 

 fresh-water or brackish-water origin. With the close of the Tertiary, and 

 probably before the Pliocene had fully passed, elevatory movements occurred 

 which raised the Tertiary of the Atlantic border about 100 feet, and that of 

 the Gulf border not much more, except along a region in Georgia, and the 

 border of Alabama in a line with the Peninsula of Florida, where the height 

 is 300 to 400 feet above sea level. A Florida axis of elevation is indicated 

 by it. On Long Island, Martha's Vineyard, and other islands south of New 

 England, occur upturned beds of the Cretaceous or Cretaceous and Tertiary, 

 indicating orogenic movements before the Quaternary. See further, page 1021. 



The elevation of the Atlantic border may have been part of a greater 

 change which affected also the whole of the Appalachian region; but no posi- 

 tive evidence of this is yet obtained. 



What was the total gain in mass through the great Tertiary elevation of 

 the North American continent ? On this point little is known with regard 

 to its eastern half, but the western affords available facts. 



With the opening of the Tertiary the larger part of the western half of the 

 United States was at the water's level from the eastern foot of the Sierra Nevada 



