416 



University of California. 



[Vol. i. 



subsidence in the middle of the Great Valley, and the subsidence of 

 the bay region, stand in close relation to each other, but are not due 

 to loading down by sediments. 



The great longitudinal depression occupied by the Gulf of Cal- 

 ifornia and the Colorado Desert, offers some very interesting analo- 

 gies with the Great Valley of California. According to Lindgren * 

 and Emmons and Merrill, f the long peninsula to the west of the 

 Gulf has substantially the structure of the Sierra Nevada, rising 

 toward the east in a very gentle slope, and then falling abruptly, in 

 a great escarpment of several thousand feet, to the narrow strip of 

 gently sloping mesa on the immediate Gulf coast. The area occupied 

 by the Gulf, and by the desert valley to the north, is accordingly 

 comparable, as regards structure, with the Basin Region to the east 

 of the Sierra Nevada, and not with the Great Valley of California. 

 But this is the more interesting, as showing a second way in which 

 a long, narrow valley can be formed independently of sedimentary 

 loading, by faulting instead of by gentle flexing of the crust. Into 

 this valley the Colorado River has poured vast quantities of sedi- 

 ment from the waste of the great interior plateaus. Entering from 

 the east side, like the San Joaquin and Kings Rivers, it has thrown 

 a great delta across the Gulf, and finally cut the latter off completely 

 from any communication with the northern portion of the valley. 

 This severed northern end thus became a lake, comparable with 

 Tulare Lake, and finally through complete desiccation was converted 

 into the great Colorado Desert of to day, portions of which are 275 

 feet below the sea. \ The recency of these events, and the present 

 activity of the processes through which they have been effected, are 

 strikingly shown by the way in which the Colorado River occasion- 

 ally makes breaches in its bank, and sends a portion of its water 

 down the northern slope of its alluvial fan, forming in the desert the 

 well-known New River and Salton Sea. Thus we have here a close 

 parallel to the process which is going on in Tulare Valley to-day. 



* Notes on the Geology of Baja California, Proc. Calif. Acad. Sci., 2 ser. , 

 Vol. I, p. i73- 



t Geological Sketch of Lower California, Bull. Geol. Soc. Am., Vol. V, 

 pp. 489-514- 



£Preston, nth Rept. State Mineralogist of Calif., p. 387. 



