Notes on Monhegan Island. — Lord. 335 
mineral resembling chlorite, which terminates abruptly in an 
outer fringe of bluish-green actinolitic hornblende. This outer 
zone is usually from iV-i vavci. thick and is formed of a dense 
interlocking mass of actinolite fibers. In many specimens, 
where the effects of orogenetic processes are more apparent, 
actinolite needles extend out in all directions, frequently whol- 
ly replacing the feldspar. 
The actinolite is strongly pleochroitic (parallel c=bluish 
green, parallel b^grass green, parallel a-"=pale yellow) and 
shows on prismadic sections a maximum extinction of- 22° (c:c 
=22°). 
In many cases the olivine is itself altered to a pilitic* mass 
consisting of fibrous tremolite. actinolite, chlorite and flakes 
of brown mica. 
The olivine is normally the first ferro-magnesian constitu- 
ent to crystallize, but it was found in some sections to contain 
large feldspar inclusions indicating a later period of crystalliza- 
tion. These inclusions are surrounded by rims of secondary 
hornblende and chlorite identical with those formed on the 
outer contact with the olivine, thus indicating that the minerals 
of both zones are of similar origin, and formed after the rock 
magma had reached a state of solidity. 
Hypersthene is a constant accompaniment of the olivine in 
the Monhegan norytes. It occurs, either in the form of irreg- 
ular grains, often surrounding the olivme, or as w^ell developed 
crystals evenly distributed through the rock. The granules 
are in some instances associated with the olivine in such a 
manner as \o suggest a resorption rim, but careful examina- 
tion leads to the conclusion that the minerals are of separate 
origin, and their proximity due simply to mechanical segrega- 
tion during the solidification of the rock. 
In the noryte proper the orthorhombic pyroxene is the pre- 
dominating ferro-magnesian constituent. It occurs here in 
well developed prismatic crystals with distinct cleavage par- 
allel (100) (010) and (no) and strong pleochroism; parallel (a) 
purplish to reddish brown, parallel (b) light brownish yellow, 
parallel (c) pale green. It is, furthermore, easily recognized bv 
the characteristic metallic luster (schillerization) produced by 
original inclusions of thin, rectangular plates of brown titanic 
*See P. Becke., op. cit., p. 355. 
