AUTHORS ABSTRACTS 249 
Placerville district, signed by Waldemar Lindgren 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 
geology, 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 contains 
925 square miles. It is located in the upper foothill region of the 
Sierra Nevada, the elevation ranging from 300 feet to 5400. The 
prevailing character of the topography is that of irregular and undulat- 
ing 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 River in the southern 
part. 
Geology.—The eastern half of the tract is principally composed of 
a somewhat metamorphosed sedimentary series, the Calaveras forma- 
tion, of presumable Carboniferous age. ‘The rocks consist chiefly of 
clay-slates and quartzitic sandstones, and have in general a northerly 
strike and steep easterly dip. Several irregular intrusive masses of 
granitic rocks are contained in the sedimentary series. ‘The western 
half of the tract is much more complicated. A belt of black slates 
belonging to the Mariposa formation, or 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 easterly dip. The western part 
of the area contains a great abundance of basic igneous rocks, con- 
sisting of diabase, augite, hornblendic porphyrite, gabbro-diorite, 
pyroxenite, and serpentine. Over large areas certain of these basic 
rocks have been converted to amphibolitic schists by dynamo-meta- 
morphic processes. Covering the ridges and resting unconformably 
on the older rocks are large masses of Neocene effusive rocks, chiefly 
tuffs and breccias of rhyolite and andesite. These masses form gently 
sloping tables, underneath which the Neocene gravel channels are 
found. 
Economic geology.—TYhe Neocene river channels, with very highly 
auriferous gravel, are exposed and mined at several places in the area, 
for instance, at Todd’s Valley, near Georgetown, and in the vicinity of 
Placerville. Many and important auriferous quartz veins are found in 
