Orbicular Gabbro. — Kcsslcr and Hamilton. 137 
This rock has generally a greenish color from the super- 
abundance of green deec imposition products of olivine and liy- 
persthene. In the held, rocks were noted with this variety of 
spheroid and with dykes of hornblendic material cutting 
through the orbicular portion, and with the hornblendic por- 
tion itself orbicular. 
The third variety is the rarest of the three. This one is of 
the same type as the one described in professor Lawson's pa- 
per. The outer zone has a distinct radial and concentric struc- 
ture (Fig. B, PI. V and PI. X). It is much wider than the 
corresponding zone in the second variet}-, being about 12 mm. 
to 15 mm. in thickness and about 60 mm. in diameter. The 
.--pheroids are best seen on a w-eathered or a polished surface. 
From an examination of the hand-specimen, the observer 
would suppose the orientation of the feldspars to be radial. 
L'pon a microscopical examination of a thin section, however, 
the radial appearance is seen to be due to radiating olivines, 
whereas no definite orientation whatever has been noted in 
the feldspars. The nucleus is made up of about equal ([uanti- 
ties of light and dark minerals, with the dark hornblende pre- 
dominating. Feldspars are found in the nucleus as inclusions 
in hornblende (Fig. A. PI. VII). Where it occurs as such it has 
a corroded surface and a roundish outline, probably showing 
that there was a partial resolution of the feldspars in the orig- 
inal fluid magma. These corroded feldspars appear snow- 
white when seen in the hand-specimen. 
The feldspars give extinction angles which vary from 2y° 
to 40°. measured from the albite lamellae. In the hand-speci- 
men they often show iridescence, showing a beautiful play of 
colors similar to that seen in the typical labradorite. Their 
average size is about 0.22 mm. in diameter. 
The olivines are long, lathe-shaped aggregates varying in 
width from 0.03 mm. to 0.20 mm. and in length from 0.8 nitn. 
to 1.4 mm. At certain intervals can be seen concentric rings 
in which the olivine is lacking (Fig. B, PI. VII). These rings 
are about 0.06 mm. in width. The olivines nearly all contain 
minute inclusions of feldspar averaging about 0.006 mm. in 
diameter. This shows that the feldspars already existed as 
such when the radial arrangement of the olivines took place. 
The olivine is verv fresh and has high interference colors. 
