IGNEOUS ROCKS OF THE SIERRA NEVADA. 4OI 
If this rule be followed, all of the quartz-porphyrites of the 
Jackson geologic map should be called quartz-porphyries. 
The following analyses, however, show that the lime and alkali 
ratios are very similar in the Jackson sheet quartz porphyries, 
and in typical dacites, so that in reality they constitute a group 
intermediate between typical quartz-porphyries and _ typical 
quartz-porphyrites. 
DACITES Quartz- PORPHYRIES 
129 131 A B 151 553 549 
SiO] oe 65.66 67.49 69.51 68.20 70.29 71.19 72.24 
CHO) aoe 3.64 2.68 1.71 4.33 2.30 2.87 3.40 
KOR.) - 2.03 2.40 3.34 1.52 3.05 1.82 39 
N,O-- - - 3.65 4.37 3.89 2.98 2.68 4.24 4.43 
Dacites 129 and 131 are from Sepulchre Mountain,’ and the 
two designated as A and B are from Lassen Peak, Cal.? 
Nos. 151, 553 and 549 are from the area of the Jackson 
sheet. 
These rocks with porphyritic quartzes seem usually to be 
intrusive in more basic igneous rocks, augitic porphyrites and 
amphibolite-schist. An exception to this rule is the rock of the 
south part of the Gopher Ridge, south of the Salt Spring Val- 
ley reservoir. This grades over into a porphyrite containing 
augite, and is probably older than quartz-porphyries 553 and 
549 which occur at the west base of the range, and are almost 
certainly intrusive, and, as no analysis has been made of it, it 
should perhaps be properly called a quartz-porphyrite and not a 
quartz-porphyry, on the basis above stated. 
Intrusive in the augite-porphyrite and tuff that forms the 
western ridge of the Bear Mountains is a dike of serpentine, 
originally a peridotite or pyroxenite. 
We have then in the Jackson sheet area: 
INTERMEDIATE - Augitic porphyrites — Effusive. 
Basic - - - -  Peridotite — Intrusive. 
AcID - - - - Quartz-porphyry — Intrusive. 
« Thirteenth An. Rep. U. S. Geol. Survey, p. 648. 
? Bull. No. 9, U.S. Geol. Survey, p. 16. 
