336 The American Geologist. December, 19(0 
iron ore, arranged parallel to the brachypinacoid (010), with 
their longer sides coinciding with the axis a of the hypers- 
thene. These inclusions are identical in origin with those in 
the hypersthene of the Baltimore gabbros, as described by 
Williams* and cannot be considered as products of decom- 
position.! The hypersthene is not infrequently surrounded by 
a fringe of uralitic hornblende, which in many cases, replaces 
the entire crystal, and is optically hardly distinguishable from 
the actinolite resulting from the olivine. 
A distinctive mineralogical feature of the Monhegan rocks 
is the subordinate position occupied by the monoclinic py- 
roxene. In the noritic types it is usually absent or distrib- 
uted sporadically in the form of irregular grains heavily 
charged with magmatic inclusions. It is in the gabbroitic 
facies only that this mineral assumes larger proportions. The 
greenish brown, thick, tabular crvstals are of irregular out- 
line, and but slightly pleochroitic. They show well developed 
prismatic cleavage and the orthopinacoidal parting character- 
istic of diallage. On sections parallel to the plane of symmetry 
these crystals extinguish at an angle (c:c) of about 38° with the 
trace of the prismatic cleavage. The diallage is replete with 
ferritic inclusions, quite similar in character to those of the 
hypersthene, and is in general considerably uralitized. 
Magnetite and apatite are common in the Monhegan rock 
types — the former fluctuating quantitatively in direct propor- 
tion to the bi-silicates. 
Apart from the fibrous aggregations of uralitic hornblende 
and actinolite already mentioned, the microscope reveals in 
nearly every instance small grains of a brown, primary horn- 
blende. This mineral can best be described in the hornblende- 
gabbro which forms mineralogically a connecting link between 
the olivine-noryte and gabbro-dioryte. The rock is dark gray, 
almost black, and porphyritic in structure. Pheno- 
crysts of bytownite and hornblende are embedded in a dense, 
coarse-grained ground mass composed of hornblende, diallage, 
hypersthene, bytownite, and magnetite in about equal propor- 
tions, and all, excepting the hornblende, pan-idiomorphically 
*Bull. 28, U. S. G. S., 1886, pp. 23-24. 
fSee J. W. Judd: Teritary Peridotites of the Western Islands of 
Scotland, Q. J. Geol. Soc, Vol. 41, p. 383. August, 1885. 
