CRYSTALLINE ROCKS OF OAK. HILL AREA 159 
Common augite occurs in considerable quantity in the norite, but 
scarcely any is present in the gabbro. The individuals are allotrio- 
morphic and very irregular in shape, but show the usual cleavage 
angles of 87°. The pleochroism of the augite is from a colorless to 
a light brown. It may often be seen perfectly twinned. Some 
inclusions of olivine were noticed. 
Olivine is a constant constituent of all the gabbros. In ordinary 
light it is colorless, while in polarized light it exhibits the characteristic 
high relief and strong double refraction. ‘The crystals are usually 
rounded, but are generally much weathered and blurred. It is often 
serpentinized along irregular cracks that ramify through the crystals. 
Magnetite is found in small opaque patches scattered through all 
the sections of gabbro and of norite. 
Relations of the gabbros.—From a study of the field relations and 
the examination of a number of thin sections it is evident that the 
different variations of gabbro are merely local. Some of the gabbros 
have much olivine—olivine gabbro; some have much hornblende— 
hornblende gabbro; many of the gabbros have abundant hypersthene 
—hypersthene gabbro or norite; many have a predominance of dial- 
lage—normal gabbro. 
The typical norite from the cut at the top of Quarry Hill contains 
much hypersthene, together with some hornblende and augite, while 
it contains little olivine and diallage. On the other hand, the typical 
gabbro contains much diallage, but practically no hypersthene, 
hornblende, nor common augite. Sections have been examined 
which show almost perfect gradations between these various kinds 
of gabbro, thus strongly suggesting that they are phases in the differ- 
entiation of the same magma. 
Those gabbros which are rich in diallage and poor in hypersthene, 
hornblende, and augite most nearly approach the pyroxenites, next to 
be described, in constitution. Those which are rich in hypersthene, 
hornblende, and augite, but poor in diallage, most nearly approach 
the diorites. Thus the gabbros which undoubtedly represent the 
most acid end of the interesting basic series of pyroxenites, peridotites, 
and resulting serpentines may also be a connecting link between 
these and the more acid diorites. All the above gradations may be 
observed within a radius of seventy-five yards of Quarry Hill. 
