716 



SCIENCE 



[N. S. Vol. II. No. 



western border and another area east of 

 Boulder Eiver. 



In the Crazj' Mountains the igneous 

 rocks are wholly intrusive. The extrusive 

 rocks are andesitic brecciaj acid and basic ;, 

 trachytic rhyolite and basalt. The intrusive 

 rocks are gabbro, diorite,thera.lite, basic and 

 acid porphyrties, basic and acid andesites, 

 and dacites. Several centers of volcanic 

 eruptions, active in early Tertiary time, 

 occur in the region. They are at Emigrant 

 Gulch, Haystack Mountain and Crazy 

 Mountains. Other centers are just outside 

 of the limits of the atlas sheet. 



The chief economic deposits of the dis- 

 trict are the gold-bearing gravels of Emi- 

 grant, Bear and Crevice Gulches. They 

 have been worked on a small scale. Gold 

 veins occur in Emigrant Gulch, Crevice 

 Gulch and Haj^stack Mountain. Copper 

 ores in small quantities have been found at 

 the head of Boulder River and of Slough 

 Creek. Claj'S serviceable for brick-build- 

 ing occur in the alluvium near Livingston 

 and in the lake beds near Bozeman,also in 

 the Cretaceous strata. Two coal fields 

 exist within the district, the Cinnabar field 

 and the Bozeman field. The aggregate thick- 

 ness of the seams is from 12 to IS feet, 

 made up of a number of seams, only three 

 of which are workable. The coal is bi- 

 tuminous, of variable character, and in 

 places is a fair coking coal. The output 

 in 1889 was 49,400 tons. 



FOLIO 3, PLACERVILLE, CALIFORNIA, 1894. 



This folio consists of 11 pages of text 

 descriptive of the Gold Belt and I-?? pages 

 descriptive of the Placerville district, signed 

 by AValdemar Liudgren and H. W. Turner, 

 geologists, and G. F. Becker, geologist in 

 charge; a topographic map (scale 1:125,000) 

 of the district, a sheet showing the areal 

 geologj^ another showing the economic 

 geology, and a third exhibiting structure 

 sections. 



Geography. — The territory represented lies 

 between the meridians 120° 30' and 121° 

 and the parallels 38° 30' and 39°, and con- 

 tains 925 square miles. It is located in the 

 upper foothill region of the Sierra ISTevada", 

 the elevation ranging from 300 feet to 

 5,400. The prevailing character of the 

 topography is that of irregular and undula- 

 ting plateaus, cut by deep canyons and steep 

 ravines. The district is drained by the 

 three forks of the American River in the 

 northern part and by the three forks of the 

 Cosumnes Eiver in the southern part. 



Geology. — The eastern half of the tract is 

 principally composed of a somewhat meta- 

 morphosed sedimentary series, the Calave- 

 ras formation, of presumable Carboniferous 

 age. The rocks consist chiefly of clay 

 slates and quartzitic sandstones, and have 

 in general a northerlj' strike and steep east- 

 erly dip. Several irregular intrusive masses 

 of granitic rocks are contained in the sedi- 

 mentary series. The western half of the 

 tract is much more complicated. A belt of 

 black slates belonging to the Mariposa for- 

 mation, of late Jurassic age, traverses the 

 tract from north to south. To the west of 

 this belt follow again sedimentary rocks of 

 the Calaveras formation, greatly cut up by 

 igneous rocks. The sedimentary rocks 

 here, as well as in the western part, have a 

 northerly strike and steep easterlj^ dip. 

 The western part of the area contains a 

 great abundance of basic igneous rocks, 

 consisting of diabase, augite, hornblendie 

 porphyi-ite, gabbro-diorite, pyroxenite and 

 serpentine. Over large areas certain of 

 these basic rocks have been converted to 

 amphibolitic schists by djmamo-metamor- 

 phic processes. Covering the ridges and 

 resting uucouformably on the older rocks 

 are large masses of Neocene efi'usive rocks, 

 chiefly tufls and breccias of rhyolite and 

 andesite. These masses form gently sloping 

 tables, underneath which the Neocene 

 gravel channels are found. 



