250 A. WALL OR Se @AB Sie GAs: 
the area. The principal ones occur along the belt of Mariposa slates 
previously mentioned, and form the northern end of what is usually 
referred to as the Mother Lode of California. Passing by Nashville 
and Placerville, the vein is almost continuous up to the northern part 
of the area, where it splits up into several branches, which die out 
before reaching the northern border. Important veins are, however, 
also found both to the east and west of this belt. Near the eastern 
line lies the important mining district of Grizzly Flat. 
There are practically no alluvial soils in the area. The deep soil 
on the summit of the ridges is always a residual soil, formed by the 
decomposition of the rocks in place. 
U.S. Geologic Atlas, Folio 5, Sacramento, California, 1849. 
This folio consists of one and one-half pages of text descriptive 
of the Gold Belt and one and one-half pages descriptive of the Sac- 
ramento tract, signed by Waldemar Lindgren, geologist, and G. F. 
Becker, geologist in charge; a topographic map (scale 1 : 125,000) of 
the tract, a sheet showing the areal geology, another showing the 
economic geology, and a third exhibiting structure sections. 
Topography.—The Sacramento tract includes the territory between 
the meridians 121° and 121° 30’ and the parallels 38° 30’ and 39°, 
and contains 925 square miles. The western half of the tract embraces 
a part of the Sacramento Valley, while the eastern half contains the 
first foothills of the Sierra Nevada. ‘The elevation ranges from 30 feet 
above sea level at Sacramento to 2100 feet in the northeastern corner 
of the tract. The foothill region forms a sloping and undulating 
table-land, through which the American River has cut a deep and 
narrow canyon. 
Geology.—A small area of sedimentary slates of the Calaveras 
formation (Carboniferous) occurs in the northeastern corner, and a belt 
of black clay-slates belonging to the Mariposa formation (late Jurassic) 
is contained in the igneous rocks of the southeastern part. At Fol- 
som the Mariposa slates are cut off and contact metamorphosed by the 
granitic rocks of the Rocklin massif. The larger part of the older 
rocks of this tract is of igneous origin. A large area of diabase and 
porphyrite is found along the eastern margin. Wide belts of these 
rocks have been rendered schistose and changed to amphibolites by 
dynamo-metamorphic processes. Several masses of granodiorite and 
gabbrodiorite have been intruded into the diabases, porphyrites, and 
