210 Tlie American Geologist. October, 1900 
The mineral occurs as an endogenetic decomposition pro- 
duct of labradorite. 
Penninite is lamellar in habit, with perfect cleavage. The 
color is pale green with a weak pleochroism as follows: 
«g ■== pale green 
»m = pale green 
«p = pale yellowish green to colorless 
Absorption wg = nm < 
'P- 
The refringence is somewhat higher than that of labra- 
dorite, the birefringence is very weak, and often scarcely per- 
ceptible. Penninite is biaxial and negative in the section ex- 
amined, but the optic angle is very nearly 0°. The dispersion 
is strong, giving the "ultra blue" and pale yellow colors on 
either side of the position of the extinction, which is not com- 
plete. 
The penninite of Pigeon point often contains pleochroic 
halos. It is an alteration product, both endogenetic, and ex- 
ogenetic, from labradorite; and is often formed by exogenesis 
from biotite, hornblende, and augite. 
ClinocJilore is very similar to the preceding; it differs from 
it in possessing a notable birefringence, at least .010. It pos- 
sesses the same color and pleochroism, but with: 
«g = pale yellowish green to colorless 
fim = pale green 
«p = pale green 
Absorption : ;/g < wm < n\> • 
The clinochlore, in sections from Pigeon point, is positive, 
with a variable, but rather small optic angle. The dispersion 
is not noticeable. The elongation is negative, with extinction 
sensibly parallel. In a section from Pigeon point, clinochlore 
shows marked pleochroic halos, in one case about magnetite. 
(See Plate XVII, Fig. 5.) 
Hornblende is a common product of the alteration of the 
pyroxene. It is usually compact, and scarcely ever passes 
through the fibrous condition before altering to chlorite or 
biotite. It shows good prismatic cleavage at an angle of about 
124°; twinning is rare. The color is either brown or more 
