Wes AGE, LUNID) SUCCESSION Ol Wels, WEIIZOWS 
IROCIKS, OF Was, SIUEIRIVA INIBW AIDA. 
TuE following notes and suggestions on the age-of the igne- 
ous rocks of the Sierra Nevada are the outgrowth of field work 
in two parts of the range; one portion being a strip thirty miles 
wide across the mountains just south of the fortieth parallel, 
chiefly in the area of the Chico, Bidwell Bar and Downieville 
sheets of the United States Geological Survey ; the other portion 
that part of the range to the south of Cosumnes River, or approxi- 
mately latitude 38° 30’ N. The succession has been made out 
only partially and for only certain districts, and the article should 
therefore be considered merely as a contribution to the subject 
which must be later revised, and can be fully treated of only 
after more field work and a more thorough study of the material 
now on hand. 
Mr. Waldemar Lindgren has acquired a large amount of 
information concerning the district across the central part of 
the range from Sacramento to Lake Tahoe, the geological maps 
of which have been prepared under his direction, and much light 
may be expected when his results are published. The writer is 
indebted to Mr. Lindgren for friendly criticisms. 
The Sierra Nevada as a topographic unit may be described, 
following Whitney,’ as the mountain area lying to the east of 
the Great Valley of California and to the west of the Great Basin, 
extending from near the Tejon Pass at the south end of the 
Great Valley to Lassen Peak on the north. As thus defined, the 
* Published by permission of the Director of the U. S. Geological survey. 
The chemical analyses given in this paper, unless otherwise stated, have been 
made by Dr. W. F. Hillebrand of the U. S. Geological Survey, whose high standing as 
an analyst is well known. 
* Auriferous Gravels of the Sierra Nevada, p. 7. 
385 
