Septembee 2, 1898.] 



SCIENCE. 



293 



surrounded by high mountain barriers, with 

 diflferent zones of temperature and varying 

 degrees of humidity. 



As brought out by the labors of Le Conte, 

 Diller and Lindgren, the Sierra Nevada 

 region has undergone cycles of denudation, 

 and these changes, occurring later than 

 those of the Appalachian region, have 

 doubtless had much to do with the present 

 diversified and variable fauna. The latest 

 writer, N. F. Drake,* states that the west- 

 ern slope of the Sierra Nevada " was prob- 

 ably once a region worn down almost to 

 base-level or to a peneplain. By the uplift 

 of the mountains a great fault was devel- 

 oped along the eastern face and the whole 

 Sierra crust-block tilted to the westward. 

 The streams quickened by the uplift again 

 set to work on the peneplain and carried it 

 to its present condition." 



Le Conte f states that the Sierra Nevada 

 was upheaved at the end of the Jurassic 

 period. This corresponded to the Appa- 

 lachian revolution which occurred at the 

 end of the Paleozoic era. 



" But during the long ages of the Creta- 

 ceous and Tertiary this range was cut down 

 to very moderate height. * * * The 

 rivers bj' long work had finally reached 

 their base-levels and rested. The scenery 

 had assumed all the features of an old to- 

 pography, with its gently flowing curves * 

 * * At the end of the Tertiary came the 

 great lava streams running down the river 

 channels and displacing the rivers ; the 

 heaving-up of the Sierra crust-block on its 

 eastern side, forming the great fault-cliff 

 there and transferring the crest to the ex- 

 treme eastern margin ; the great increase 

 of the western slope and the consequent 

 rejuvenescence of the vital energy of the 

 rivers ; the consequent down-cutting of 

 these to form the present deep canyons and 



* The Topography of California. Journ. of Geology, 

 v., Sept.-Oct., 1897, p. 563-578. 



■\ Bulletin Geol. Soe. Amer., 11., pp. 327, 328. 



the resulting wild, almost savage, scenery 

 of these mountains." 



This view is further carried out by J. S. 

 Diller, from his studies of the northern 

 part of the Sierra Nevada, including the 

 borders of the Sacramento valley and the 

 Klamath Mountains. He shows that nor- 

 thern California, during the earlier portion 

 of the auriferous gravel period, was by long 

 continued degradation worn down to base- 

 level conditions. " The mountain ranges," 

 he says, " were low, and the scenery was 

 everywhere characterized by gently flowing 

 slopes." * * * 



" The topographic revolution consisted in 

 the development out of such conditions of 

 the conspicuous mountain ranges of to-day. 

 The northern end of the Sierra Nevada has 

 since been raised at least 4,000 feet, and 

 possibly as much as 7,000 feet, and a fault 

 of over 3,000 feet developed along the east- 

 ern face of that portion of the range."* 



According to Lindgren the Sierra Nevada 

 was eroded to, or almost to, a peneplain 

 during Cretaceous times, and the moun- 

 tains elevated in a later Cretaceous period 

 were worn down during Tertiary times 

 merely to a gentle topography. 



The other post- Cretaceous changes of this 

 vast region are thus summarized by Scott 

 from the results of Pacific coast geologists. 

 In the Eocene a long narrow bay occupied 

 the great valley of California, extending 

 northward into Oregon and Washington. 

 At the end of the Eocene or early in the 

 Miocene an elevation in California shifted 

 the shore line far to the west. In the Mio- 

 cene the Coast Range formed a chain of 

 reefs and islands, and at the close an up- 

 turning and elevation of the mountain 

 range took place, though it became higher 

 afterwards. The Coast Range sank again 

 early in the Pliocene and the San Francisco 

 peninsula was an area of subsidence and 



*14th Ann. Kept. U. S. Geol. Survey, Part II., p, 

 433. 



