Orbicular Gabbro. — Kesslcr and Hamilton. 135 
in thickness are seen in many places, made up almost entirely 
of black hornblende crystals. Some of these hornblendes 
were noted nearly' four inches in length. They seem to be well 
distributed over the gabbro area. 
A peculiar wavy banding is often seen in the gabbro b.)ul- 
decs. It is made up of alternate layers of light and dark min- 
erals, the band usually continuous in width, despite its undulat- 
ing arrangement. This appears to be llnxion structure. 
The principal constituents of the gabbro are : plagioclase, 
hornblende, olivine, hypersthene and oxide of iron. 
The feldspar usually predominates, though the hornblende, 
owing to its resistance to weathering, is the most prominent 
on the weathered surface. The extinction angles of the feld- 
spar vary 26° to 40°, measured from albite lamellae. Twinning 
after the albite and ^cricline laws is common. The twins of- 
ten show evidence of crushing — fraying out or ending abrupt- 
ly. Iir some hand specimens the feldspar contains inclusions 
of olivines. In others the olivines contain the feldspars as in- 
clusions. The former occurrence is the more frequently noted. 
The hornblende is of the brown basaltic type. Extinctions 
c Ac as high as 16° have been noted. It has a strong pleo- 
chroism, brown to yellowish. A cleavage angle of 123° was 
noted in the hand specimen. 
Olivine is usually very fresh, but in thin sections taken 
from near thq surface a great variety of decomposition pro- 
ducts are seen. Hornblende, actmolite, tremolite, chlorite, 
iddingsite, serpentine, limonite, and an unidentified member cf 
the sodalite group, were noted as secondary to olivine. 
Hypersthene is usually partially uralited, enough of the 
original hypersthene usually being left to show the origin of 
the amphibole. In one section the hypersthene was almost 
completely enveloped in a network of magnetite. In the same 
section the olivines are also seen to have similar inclusions. 
This phenomenon was probably caused by the presence of a 
superabundance of iron when the ferro-magnesian mineral 
crystallized. The pleochroism of the hypersthene is distinct, 
green to pink. 
Diallage is almost entirely absent. In only one thin section 
was it noted and in that its identity was not very clearlv es- 
tablished. 
