Notes on Monhegan Island. — Lord. 339 
the other made up of pyroxene with some little bytownite, 
magnetite and apatite (gabbro-pyroxenyte). Bands of ex- 
tremely coarse grained noryte occupy mineralogically and 
geologically an intermediate position between the two. These 
dike-like masses have an irrregular lenticular form without 
persistency in strike or dip. They can rarely be followed for 
more than 20-30 yards, and terminate generally in narrow 
veins and stringers while merging laterally into the surround- 
ing rock without definite planes of contact. 
The mineralogical interest of these rocks is confined chiefly 
to the pyroxenyte, which is of a dark gray color in contrast to 
the lighter purplish gray of the feldspar aggregates. 
The pyroxene of the one dark vein critically examined is 
a brownish green augite resembling diopside. It is of crude 
prismatic habit and has well defined cleavage parallel (no). 
It is furthermore characterized by the unusual position of the 
optical constants; the axis of greater elasticity (a) being nearer 
the crystallographic axis c (parallel to the cleavage) than that 
of least elasticity; as is generally the case A a:c=about 38"" in 
maximo. Dispersion of the bisectrices is recognizable: the 
angle c : a < c:a- Pleochroism is strong: parallel C apple 
p V 
green parallel b greenish yellow, parallel a purplish brown. 
Absorption: a > b > c • 
A diopside-like pyroxene of almost identical physical prop- 
erties, has been described by Broegger* from some of the 
coarse grained dikes near Frederiksvarn, Norway, but there 
the mineral appears to grade into aegirine, which is not the 
case in the Monhegan dike. Microlitic inclusions of opaque 
ilmenite needles, arranged crystallographically very much like 
those in the bytownite, are very plentiful in this pyroxene. 
The chemical relationship of the rocks just described is 
shown by the following analyses: 
I. Olivine-noryte.f 
*W. C. Broegger: Min. der sudnorweg. Augitsyenite. Zeit. fur 
Kryst., 1890, Vol. 16, p. 656. 
tin order to secure inaterial representing as nearly as possible the 
average composition of the rock-types described under this heading. 
6 specimens were selected that united yielded a mass composed (by 
volume) of approximately 60 per cent bytownite, 20 per cent olivine, 
10 per cent hypersthene. 10 per cent actinolite-(-diallage-f magnetite-|- 
apatite. 
