Northern California and its peculiar Lavas. 49 
basalt at the Cinder Cone and like it is the youngest lava of 
that vicinity. 
Grains of quartz have been noticed by other observers in 
basalts from various parts of the world, but they have been 
almost invariably considered to be inclusions of entirely for- 
eign quartz picked up at the time of the eruption. 
The great difficulty which has stood in the way of consider- 
ing the quartz as indigenous to the basalt is of a chemical 
nature, and on account of it, petrographers generally are disin- 
clined to admit that it is possible for quartz to crystallize in a 
magma which yields olivine. It must be borne in mind, how- 
ever, that the quartz and the olivine did not crystallize syn- 
chronously nor under the same conditions. The quartz is 
older than the olivine and crystallized under great pressure far 
beneath the surface in that region where the conditions of phys- 
ical and chemic equilibrium are as yet so largely conjectural. 
The crystallization of the olivine was rendered possible by the 
earlier secretion of free silica. 
Chemical Analysis of quartz basalt from the Cinder Cone. 
Sie ihe AC Ur tale base a ee Os 
Ai ON ayaa UA cya) Meine ee ea Os60) 
BS OS as CCG Ske eae 16°45 
IB CR@) ire AOR A tds AE: Sep 1°67 
1 ahs O ait eae tse gets Tata egy Seed manatene We che. Way) 
ANY pay © ances eS eee Ol ie, inl ers ANTS 0°10 
(OEY OR Seo Ie le alten etc al A  S izant 7°65 
TO) ae vires NOR eee it Ua Ure trace 
| BO) Nee SI a is Pas A ala eh ga Su 0:00 
ANE 0) Wi ine ena nays ree etree A) ake 6°74 
1 SS ahs a ad re nod ia aay 9 ish 
Bt EA Gale ls aN pea elo a Ace A 3°00 
Li, OP DREN CaN vont ie eas eye | 0:00 
IEEE OUS Apa RING 5a Ls bel 0°40 
LEM OW en ON Get heed ee 0-20 
100°35 
The accompanying chemical analysis of the quartz basalt 
from the Cinder Cone near Snag Lake has been made for me 
in the labo atory of the U. 8S. Geological Survey by W. F. 
Hillebrand.t The high percentages of silica and magnesia are 
to be noticed in connection with the low percentages of the 
oxides of iron. 
Corroded quartz is abundant in the quartz porphyries and 
rhyolites, less abundant in the andesites and rare in the basalts, 
nevertheless its presence is an essential feature of the basalt in 
which it occurs and may be used as one of the means of dis- 
AM. JOUR. 5 ea SERIES, VOL. XXXIII, No. 193.—January, 1887. 
