Notes on Monhegan Island. — Lord. 33 
The crystals cleave readily parallel P (001) and M (010), 
and show on these faces average extinction angles of 
about 24 and 33 degrees respectively. They have a specific 
gravity of 2.^2, which, with the properties already given, 
would identify them as bytownite (Ab, Ano). 
The rather high specific gravity (Ab Anc;=2.7i normally) 
may be partially due to abundant inclusions, which frequently 
occur in such numbers as to render the crystals almost opaque 
in transmitted light, and gives them a purplish gray color and 
sub-metallic luster. 
These inclusions are either in the form of a fine, ferritic 
dust or of delicate needles arranged according to crystallo- 
graphic directions in the feldspar (parallel a b c) very like 
those described by Chester.* 
The bvtownite decomposes readily to calcite and kaolin. 
The chemical analysis of this feldspar (I) compared with 
one of bytownite from Mount Hope. Md. (Iljf shows a re- 
markable coincidence in composition. It also differs but 
slightly from the theoretical composition calculated b}' Schu- 
ster (Miner, and petrog. Mitth., III. p. 153 1881) for a mix- 
ture of six anorthite molecules with one of albite (HI). 
I II III 
SiOl- 48.06 46.17 46.5 
ALO3 32.54 35-22 34.6 
CaO 17.51 16.29 173 
NajO 1.77 2.31 1.6 
Total 99.88 100.00 100.00 
The next mineral in order of importance is olivine. It is 
in varying proportions in the olivine-gabbro and noryte, and 
forms the sole ferro-magnesian component of the troctolyte. 
It is perfectly colorless and occurs in the form of irregular, 
allotriomorphic grains common to basic plutonics. In manv 
specimens cleavage parallel (010) and (100) is well developed, 
and not infrequently the crystals are intersected by microscop- 
ic fracture planes along which magnetite is extensively devel- 
oped. 
*P. D. Chester: The Gabbros and Associated Rocks in Delaware. 
Bull. 59 U. S. Geol. Survey, p. 13-14. 
tSee G. H. Williams: The Gabbros and Hornblende Rocks, oc- 
curing in the Neighborhood of Baltimore, Md., Bull. No. 28, U. S. 
Geol. Survey, p. 20. 
