322 Mr. J. A. Phillips's Notes on the Chemical Geology 



bourhood of theTejon Pass, and extending through the State to 

 its northern limit. In consequence, however, of various local 

 circumstances, different portions of this band are of very unequal 

 importance as gold-producing districts. 



The principal auriferous region may be said to occupy the 

 western portions of the several counties of Mariposa, Tuolumne, 

 Calaveras, Amador, El Dorado, Placer, Nevada, Sierra, and 

 Plumas, with portions of the eastern sides of Yuba and Butte 

 counties. 



The apparently limited extent of the auriferous belt towards 

 the north, in the counties of Plumas and Butte, is, according to 

 the State Geological Survey, not owing to the thinning out of 

 the gold-bearing formation in these localities, so much as to its 

 being here, as well as in Shasta and Siskiyou counties, in a great 

 measure covered by large masses of lava of very recent origin. This 

 has been poured forth from Lassen's Butte and other volcanic 

 cones in its vicinity ; and, overflowing the older slates, it has 

 covered them to a great depth with a non-metalliferous and 

 almost indestructible capping. 



Beyond Mariposa, in the southern portion of the gold-region, 

 the slates are narrower and subject to interruption, and, from 

 being more frequently and more extensively encroached on by 

 the granite, they almost cease to form a continuous belt. This 

 gradual decrease in the width of the auriferous formation from 

 north to south, and the continuously increasing amount of me- 

 tamorphism displayed, are very marked, since the granite progres- 

 sively occupies a relatively larger portion of the Sierra, and by 

 degrees descends lower down its flank. 



The slates of the auriferous belt of California have been satis- 

 factorily shown by Professor Whitney "* to belong, for a great 

 extent, to the Jurassic period, although the occurrence of nume- 

 rous Triassic fossils in the gold-bearing rocks of Plumas county 

 and elsewhere renders it more than probable that no inconsi- 

 derable portion of the slates in the heart of the gold-region are 

 of that age. The sedimentary rocks of the great auriferous belt 

 lying on the western slope of the Sierra Nevada principally con- 

 sist of various slates and schists, sometimes containing nodules 

 of white felspar, which is generally more or less decomposed. 

 Among them are also found sandstones of various degrees of 

 fineness (often transformed into quartzites), black talcose schists, 

 with slates exhibiting a well-defined cleavage and silky structure, 

 together with bands of crystalline limestone. 



The rock constituting the principal mass of the Sierra Nevada 

 is a granite containing only a small proportion of quartz, and in 

 which but one species of felspar (oligoclase) is generally found. 

 * Geological Survey of California. 



