Dkcembee 13, 1895.] 



SCIENCE. 



787 



GEOLOGIC ATLAS OF THE UNITED 



STATES {II.). 



FOLIO 11, JACKSON, CALIFORNIA, 1894. 



This folio consists of 2 pages of text de- 

 scriptive of the Gold Belt, concluding with 

 a generalized section of the formations of the 

 Gold Belt, 4 pages of text descriptive of the 

 Jackson tract, signed by H. W. Turner, 

 geologist, and G. F. Becker, geologist in 

 charge ; a topographic map of the Jackson 

 tract (scale 1:125,000), a sheet showing the 

 areal geology, and a third of structure sec- 

 tions. 



The area covered by the folio embi-aces a 

 portion of the foot hUls of the Sierra Nevada, 

 chiefly in the counties of Amador and Cal- 

 averas, California. The area is di-ained by 

 the Mokelumne and Calaveras rivers. The 

 region is one of great economic importance, 

 and comprises a portion of the rich belt 

 of gold-quartz mines known as the Mother 

 Lode. One of these mines, the Utica, at An- 

 gels' Camp, is said to be paying about one 

 million dollars yearly at the present time. 



There are two distinct series of formations 

 represented in this area. The Calaveras 

 and Mariposa formations, of sedimentary 

 origin, and the associated igneous rocks 

 form an older, highly disturbed series, on 

 which a later series rests with a marked un- 

 conformity. This later series represents 

 the Tertiary and Pleistocene periods. 



The Calaveras formation, of Carboniferous 

 age, is composed of slates, quartzite, mica- 

 schists and limestone lenses, and contains 

 frequent gold-quartz veins. The Mariposa 

 formation, of Jurassic age, is largely made 

 up of clay slate. There are two main belts 

 of this formation, and in the eastern one 

 occur many of the gold-quartz mines of the 

 Mother Lode. 



The igneous rocks associated with the 

 Calaveras and Mariposa formations are of 

 considerable variety, but only three form 

 areas of great extent. These are serpentine, 

 granite and the porphyrites (old andesites) 



and their tuffs. The serpentine is undoubt- 

 edly an altered form of basic igneous rocks 

 (pyroxenite and peridotite) , and is intrusive. 

 The granite is likewise intrusive, cutting 

 through all the older rocks except the 

 Mariposa formation, and there is little doubt 

 that it is later than this formation also, and 

 in adjoining districts it invades the Mari- 

 posa slates as well. The porphyrites are 

 largely altered forms of surface lavas and 

 tuffs, resembling andesite and in part be- 

 salt, and these rocks, have been folded and 

 compressed along with the sediments of the 

 Calaveras and Mariposa formations. The 

 areas called amphibolite schists on the geo- 

 logical map are chiefly metamorphic forms 

 of these porphyrite tuffs. 



The formation of the later series, rest- 

 ing on these older rocks, that deserves most 

 attention is called the Auriferous gravel 

 formation. These gravels, which are found 

 chiefly on the ridge tops, were deposited in 

 Neocene time by rivers. These old streams, 

 as may be seen by inspecting the map, 

 united into one trunk a little to the north 

 of the Bear Mountains, and there found an 

 outlet into the gulf that then filled the San 

 Joaquin Valley. At many localities these 

 old river gravels have been profitably 

 mined for gold. Forming a capping to the 

 gravels are usually beds of volcanic ma- 

 terial, chiefly andesite and rhy elite. 



The Calaveras formation is of economic 

 importance as containing frequent gold- 

 quartz veins and lenses of limestone. Most 

 of the latter are noted on the geological 

 map. 



The Mariposa formation affords a good 

 roofing slate, but is chiefly remarkable as 

 containing, in Amador county and in the 

 north portion of Calaveras county, the 

 quartz veins of the Mother Lode. 



The amphibolite-schist belts contain cop- 

 per deposits and gold-quartz veins. In the 

 southern part of Calaveras county, at An- 

 gel's Camp, the Mother Lode lies to the 



