GINA OVS (> LAB) AHOA COTES, 975 
topographic map of the district, a sheet showing the areal geology, 
and another showing the economic geology. 
The Lassen Peak district is situated in northern California, between 
the Sacramento Valley and the Great Basin, and adjoins the northern 
end of the Sierra Nevada. It is bounded by the r2ist and 122d 
meridians and the goth and 41st parallels, and contains an area of 
3634.4 square miles. 
Within the district there are three distinct topographic features. 
Beginning at the west, it includes (1) a small portion of the eastern 
border of the Sacramento Valley; (2) the Lassen Peak volcanic ridge; 
and (3) upon the east a portion of the Great Basin platform. 
Twenty-two geological formations are shown upon the map. Thir- 
teen of these were deposited by water as sedimentary rocks. The 
remaining nine are of igneous origin, and were erupted from the 
interior of the earth in a molten condition. Some of the sedimentary 
rocks, especially the younger ones, have not been materially changed 
since they were deposited; but others, such as the auriferous slates, 
have been greatly altered or metamorphosed, and contain veins of 
quartz and metalliferous deposits. 
By far the most abundant rocks of the Lassen Peak district are 
those of igneous origin. ‘The numerous volcanoes of the district have 
furnished a great variety of such rocks. 
Beds of unaltered stratified rocks, none of which are older than the 
Cretaceous, are still nearly horizontal; although uplifted, they have not 
been compressed enough to produce folds. On the other hand, the 
auriferous slates have been thrown into a series of anticlines and syn- 
clines, and so greatly compressed as not only to close the folds, leaving 
the strata in many cases approximately vertical, but also to break and 
displace them along a series of thrust faults during the earth move- 
ments by which the mountains were produced. 
Upon the economic map special attention is called to the distribu- 
tion of auriferous slates, in which alone there is any probability of dis- 
covering valuable deposits of precious metals. These rocks are exposed 
in the southeastern and northwestern portions of the area mapped, 
and extend through under the lavas of the Lassen Peak district, from 
the Sierra Nevada to the Klamath Mountains of the Coast Range. The 
broad stretch of unaltered lavas about Lassen Peak does not contain 
any appreciable amount of precious metals, and may be wholly 
neglected by the prospector. 
