GEOLOGY OF WEST COAST REGION OF UNITED STATES 

 downwarpings, however, have occurred, which 

 have let the ocean into Puget Sound, drowned the 

 mouth of the Sacramento River, forming the beauti- 

 ful San Francisco Bay, and isolated Santa Catalina 

 and other islands from the main land. 



Professor J. P. Smith has kindly revised his table* 

 summarizing the Pacific geologic record, which ap- 

 pears on page 51. 



ECONOMIC GEOLOGY. In the following para- 

 graphs I shall attempt merely to call attention to 

 those episodes of the geologic history that have 

 developed valuable characteristics in certain of the 

 rocks, or resulted in the introduction of metallic 

 substances, and note some of the more important 

 mineral belts, with a list of their economic products. 

 These belts do not lie at hap-hazard, but appear 

 as units of the giant earth pattern which is repro- 

 duced in the geological map. 



ECONOMIC PRODUCTS OF THE PRE-CAMBRIAN 

 ROCKS. The scattered pre-Cambrian rocks have 

 furnished only a small portion of the mineral pro- 

 duction of the west coast. We might expect to 

 find such products as slate, graphite, marble, etc., 

 in the metamorphosed rocks of this age, but, as a 

 matter of fact, all the production of this class of 

 substances has come from Palaeozoic and early 

 Mesozoic rocks, metamorphosed by the Jurassic 

 batholiths. A few unimportant copper and gold 

 deposits, which bear certain characteristics of great 

 age, are believed by Lindgren to be of pre-Cambrian 

 age. These include gold prospects near Hedges, 

 Imperial County, and of Whipple Mountain, San 

 Bernardino County, California. 



THE MINERAL DEPOSITS OF PRE-CRETACEOUS AGE 

 The sediments deposited in the vanished ocean of 

 the Great Basin region contain an impressive array 

 of economic products. The sedimentary rocks con- 

 sist chiefly of argillaceous and sandy material, but 

 contain great belts of limestone of late Palaeozoic 

 age and strata of basic tuffs. The limestone belts 

 west of the Sierra Nevada furnish excellent marble, 

 which is quarried in Amador, Calaveras, Mariposa 

 and Tuolumne counties. These outcrops broaden 

 towards the north, where they reappear from under- 

 neath the lava cover of northeastern California, and 

 while Shasta and Siskiyou are registered with the 

 producing counties, untouched mountains of white 

 marble, inaccessible at present, are reported in the 

 northern part of the State and continue into Oregon. 

 In southern California the small broken desert 

 ranges often expose upturned strata of limestone 



* Published in the Jour. Geol., vol. XVm (1910), p. 225. 



52 



