324 The American Geologist. May, isos 
sandstone and serpentine, but the granite was not seen in contact 
with these rocks, although they seem conformable with the lime- 
stone and schist below, in which the granite would appear to be 
intrusive. Although this suggests the granite being later than the 
Coast range metamorphic series, it can hardly be said to prove it. 
The author states on page 72: "This determines the ages of 
the two formations; and if it is a fact, as I believe it to be, that 
this granite is identical in age with that forming the great mass of 
the Sierra Nevada, the metamorphic rocks into which it has been 
intruded belong to the same series and were uplifted at the same 
time as the Sierra Nevada. " 
There is a specimen of the Gavilan granite and one of the gneiss 
in the collection of the United States Geological Surve}- obtained 
there by the writer. This granite is very different from that of 
the Sierra Nevada. It appears to be indeed a typical granite, 
and as shown b}' a thin section is composed of plagioclase, ortho- 
clase, quartz, and biotite, while the granite of the Sierra Nevada 
is usually hornblendic with very little orthodase. The two rocks, 
however, might well be of the same age, or it is even possible that 
the Gavilan granite is older than that of the Sierra Nevada, much 
of which is bej'ond a doubt of Mesozoic age. 
It is to be regretted that Mr. Fairbanks has not brought the 
microscope to his aid in his investigations. The relation of granite 
to sedimentary rocks could perhaps in this way be made certain, 
since the intrusion of granite into sedimentary rocks often 
causes the formation of contact minerals, mica, tourmaline, gar- 
net, etc. 
The following is presented merely as a working hypothesis: 
1st. That the granite, gneiss and metamorphic limestone of the 
Gavilan range and similar areas elsewhere in the Coast ranges are 
Paleozoic and probably Carboniferous in age. 
2d. That the phthanites, hardened sandstones, and diabases, 
are earlier than the Knoxville beds. 
3d. That the serpentine, gabbro, and perhaps the glaucophane 
schist, which is frequentl}' associated with the serpentine, are post- 
Knoxville in age. 
