456 DILLER AND STANTON THE SHASTA-CHICO SEKIES. 



Jurassic, when the Sierra Nevada received its final folding and the great 

 valley of California was outlined thereby. It cannot yet be definitely 

 concluded that the Coast range and the Sacramento valley originated at 

 the close of the Jurassic; their beginnings may date from earlier foldings; 

 but whatever the date of inception, it is evident that during the Shasta- 

 Chico period the Coast range existed, but did not furnish sufficient 

 obstruction to keep the open sea out of the Sacramento valley, for the 

 fossils of that period are everywhere purely marine. That a Coast range 

 island continued in the Wallala region until the close of the Horsetown 

 epoch is indicated by the absence of the Shasta portion of the series at 

 that point. Further northw^ard, however, the waters of the Horsetown 

 epoch crossed between Bully Choop and Yallo Bally, which is now the 

 principal divide west of the Sacramento valley, and connected directly 

 with the open sea possibly in such a w^ay as to separate the island of the 

 Wallala region from that of the Klamath mountains. 



Since the above was written a brief but important paper has been 

 published by J. P. Smith,* who concludes from new faunal evidence 

 that "the Mariposa slates are of Upper Jurassic age," and that '' the 

 uplift and metamorphism of the Sierra Nevada and of the Coast range 

 occurred in late Jurassic time, before the deposition of the Cretaceous." 



SUBSIDENCE OF THE SACRAMENTO VALLEY. 



The large extent of this subsidence, from Alaska on the north to Lower 

 California on the south, makes it an epeirogenic movement. There is 

 evidence, hoAvever, that the movement, although epeirogenic, was not 

 uniform throughout the whole area. 



In the accompanying ideal section there are represented the general 

 observations made on Elder creek, Cold fork and North fork of the 

 Cottonwood, as well as those on the eastern side of the Sacramento valley 

 and at the western foot of the Coast range along the coast at Wallala, 

 and it appears that the subsidence was greater in the Sacramento valley 

 than in the region of the Coast range and Sierra Nevada. 



At the time the basal portion of the Knoxville beds was deposited the' 

 water was shallow, for there are numerous local conglomerates and sand- 

 stones, some of which are ripple-marked. If the subsidence was uniform 

 throughout the whole region it follows that what is now the western foot 

 of the Sierra Nevada, as well as the corresponding portion of the Coast 

 range, where in both cases the Chico rests directly upon the folded pre- 

 Cretaceous rocks, must have been at an elevation of over 25,000 feet above 

 the sea when the basal portion of the Knoxville was deposited in the 



* "Age of the Auriferous Slates of the Sierra Nevada." Bull. Geol. Soc. Am., vol. 5, pp. 257, 258. 



