CRYSTALLINE ROCKS OF OAK HILL AREA 163 
ently this same range of serpentine, the conditions are very similar. 
It would appear that formerly comparatively extensive mining opera- 
tions were carried on at both these places. 
Analysis of serpentine.—The analysis of this serpentine was 
furnished by the United States Geological Survey at the request of 
Dr. Branner. The other analyses of Coast Range serpentines are 
given for comparison. 
I II III 
SOS ek So oioorn oars Sea eor evo 39 .60 42.00 
Cre Feria se leet wiiertl|= 2 tie es CH2OF AE Ml crenny: 
INO ea Ge oui s CSO aE Sse 1.81 1.94 2.72 
LSA OTA Sater Couns PSRs men Reine ROMAGiG ble ee Gade aa 
He Ora ice silencer coal Tay) cae: 8.45 2.88 
INGOs Reeser sotuaeminlactacccee Roe agrees Wan Miestin Yeaneeeoe ees al irae ete ou 
VIS OES eaterapesdhes shh exshatcs disseyase 35.60 36-90 39-53 
INGO Se asnonn acess (not|determined) 
PIB) pierce ere seus cee anevtaa OOO) MG ww kere yateny callin w vac rege wneter 
MTs Oye aston ra a teceiast eee reeainy sea (not det.) 12.91 12.04 
ao tal eee Nee yes we | 85-95 100.00 99 .23 
| 
I. Massive conchoidal serpentine of the Oak Hill area, by United States Geo- 
logical Survey for Dr. Branner. 
II. Serpentine from Presidio, San Francisco; analyst, Dr. Easter. Pacific Railroad 
Reports, Vol. VI [1855]. (Pt. 2, p. 11.) 
III. Serpentine hard sound nodule in crushed matrix; analyst, Dr. F. L. Ransome. 
(“The Geology of Angel Island,” Bulletin of the Department of Geology of the University 
of California, p. 231.) 
Analysis No, 1 (United States Geological Survey) confirms the 
conclusions reached above. ‘The original rock was evidently ultra- 
basic. The large percentage of iron is doubtless accounted for by 
the presence of the mineral magnetite, while the mineral serpentine 
makes up a large part of the remainder of the rock. The proportion 
of silica to magnesium also approaches that ratio in the pure mineral 
serpentine, in which 40.42 per cent. of magnesium corresponds to 
41.52 per cent. of silica. The evidence is clear that the serpentine 
is an altered igneous rock. On some of the slopes of the Oak Hill 
area, where both sandstone and serpentine are exposed, a mechanical 
mixture of serpentine and sand has been formed. 
