DEFORMATIOXS. 455 



nearly 20,000 to 10,000 feet (estimated) on Cold fork ; the Horsetown from 

 6,100 to 5,200 feet, and the Chico from nearl}^ 4,000 to about 3,600 feet on 

 the north fork of Cottonwood creek. The thinning out is due to the fact 

 that the newer beds overlap to the northward and eastward, and it is evi- 

 dent that all the members of the Shasta-Chico series thin out to an edge 

 toward the Cretaceous land. This relation clearly indicates subsidence 

 of the land and transgression of the sea. 



To the southward also the Chico seems to be thinning, for in the region 

 of San Diego, according to Mr Fairbanks, the Chico beds containing 

 CoraUiochama orcutti, which belongs to the very basal portion of the Chico, 

 is conformably overlain by Eocene containing characteristic fossils of 

 that horizon, and the two appear to be only 1,200 feet in estimated verti- 

 cal distance apart.^ The thickness of the Chico in that region therefore 

 may not be greater than 1,200 feet, and if so, its thickness must have 

 greatly diminished to the southward. 



COAST RAXGE UPLIFT AT CLOSE OF THE JURASSIC. 



Several years ago ^Ir H. W. Fairbanks t maintained the pre-Cretaceous 

 age of the metamorphic rocks of the Coast range in California, and that 

 the Coast range with the Sierra Nevada was upheaved at the close of the 

 Jurassic. 



There is evidence favoring the same conclusion, in that the thickness 

 of the Shasta-Chico series diminishes westward from the Sacramento 

 valley into the Coast range, as it does eastward to the base of the Sierra 

 Nevada. Along the coast north of San Francisco, in the Wallala region, 

 according to Becker,^ the Chico (Wallala) beds, some thousands of feet 

 in thickness,§ rest unconformably upon the metamorphic rocks. Mr 

 Turner has studied the same region, and regards the Chico at that place 

 as unconformable not only upon the serpentine, of which the conglom- 

 erate (Chico) is said to contain pebljles, but also upon the silicious slates 

 and other pre-Cretaceous metamorphic rocks of the Coast range. The 

 relation here is probably the same as it is upon the northern border of 

 the Sacramento valley, where CoraUiochama orcutti occurs in the Chico 

 beds, which rest with conspicuous unconformity upon the metamorphics. 

 Wherever the Chico beds overlap the Horsetown their basal portion con- 

 taining the fossils of the horizon exposed near Wallala must come 

 directly and unconformably in contact with the older metamorphic rocks. 



The thinning out of the Shasta-Chico series westward from the Sacra- 

 mento valley shows that the Coast range was uplifted at the close of the 



* Am. Jour. Sci., vol. xlv, p. 476, June, 1893. 



t American Geologist, March, 1892, and February, 1893. 



JU. S. Geol. Survey, Monograph xiii, pp. 191, 19.1. 



§ While : U. S. Geol. Survey, Bulletin no. 22, p. 7. 



