A WILHORS AUPSTRACLS 
U. S. Geologic Atlas. Folio 11, Jackson, California, 1594. 
This folio consists of two pages of text descriptive of the Gold 
Belt, concluding with a generalized section of the formations of the 
Gold Belt, four pages of text descriptive of the Jackson area, signed by 
H. W. Turner, geologist, and G. F. Becker, geologist in charge; a 
topographic map of the Jackson tract (scale 1: 125000), a sheet showing 
the areal geology, and a third of structure sections. 
The area covered by the folio embraces a portion of the foothills of 
the Sierra Nevada, chiefly in the counties of Amador and Calaveras» 
California. The area is drained 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 Angel’s Camp, is said to be 
paying one million dollars yearly at the present time. 
There are two distinct series of formations represented in this area. 
The Calaveras and Mariposa formation, of sedimentary origin, and the 
associated igneous rocks form an older, highly disturbed series, on 
which a later series rests with a marked unconformity. 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 fre- 
quent gold-quartz veins. The Mariposa formation, of Jurassic age, is 
largely made up of clay-slate. There are two main belts of this forma- 
tion, 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 considera- 
ble areas. These are serpentine, granite, and the porphyrites (old 
andesites) and their tuffs. The serpentine is undoubtedly an altered 
form of basic igneous rocks (pyroxenite and pridotite), 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- 
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