CRYSTALLINE ROCKS OF OAK HILL AREA 169 
so evident that it has been found artificial to make separate classes. 
The field indications show conclusively that the pyroxenite grades 
into the typical gabbro on the one hand, and into the olivine-pyroxen- 
ite on the other. Further, this olivine-pyroxenite, which in one phase 
is practically a diallagite, in other phases goes by insensible grada- 
tions into peridotite-serpentine; that is, the phase of the pyroxenite, 
richest in olivine (peridotite) has become serpentinized and gives us 
peridotite-serpentine and serpentine. 
This serpentine, it has been suggested, gives up its iron, yielding 
veins of magnesium-silicate, which in turn liberates silica, takes up 
carbon-dioxide, forming magnesium-carbonate. ‘Thus we find over 
the area the free oxides of iron veins and patches of magnesium- 
silicate, and, where conditions favor the taking-up of carbon-dioxide, 
magnesium-carbonate. When the carbonates, silicates, free silica, 
and iron-oxides are all mixed in one complex rock, we get a product 
that has been described under the name “silica carbonate sinter.” 
The hypersthene gabbro in the field has not been observed in 
direct connection with the other rocks of the gabbro-pyroxenite- 
serpentine series; it, however, shows no evidence of intrusion. It is 
in very close proximity to the typical gabbro, and is found nowhere 
else over the area. Its mineralogical variation, moreover, would lead 
to the conclusion that it is but a phase of the ordinary typical gabbro. 
Olivine, it has been noted, is common to all this rock series, 
increasing in freshness from the serpentine to the hypersthene-gabbro. 
These rocks also have a banded structure in common. The mineral 
diallage is also common to all the rocks of the series. Its ratio to 
olivine accounts for the variations from diallagite, olivine-pyroxenite, 
peridotite-serpentine to serpentine. In acidity we have an increase 
from 37.71 per cent. of silica in the serpentine to 48.11 per cent. in 
the gabbro. 
The authors, in conclusion, wish to express their thanks to Dr. 
J. P. Smith, for his constant aid and advice; to Dr. J. C. Branner, 
for the chemical analyses secured from the United States Geological 
Survey. Mr. F. H. Tibbetts, a former student of Mr. Carey’s, aided 
materially in the construction of the topographic map. 
