198 TRANSACTIONS OF THE [apR. 16, 
Williams in the Baltimore gabbros.* Near the northern end 
the proportion of dark silicates, and especially of olivine, rap- 
idly increases, grading finally into a peridotite. It is not easy 
here to get good exposures, the ground being mostly built over 
and the rock much altered, but sufficient material can be had to 
determine with fair accuracy its limits and character. When 
unaltered it is heavy, massive and black, with a marked bronzy 
lustre from plates of hypersthene, and with irregular white ag- 
gregates of feldspar more or less abundant. When the latter 
are moderately common and regularly distributed, a remarkable 
looking rock is produced, the contrast of color being very 
strong. 
Micro-cHaRacTers. In thin sections of specimens from near 
the northern end olivine is seen to compose nearly half the 
mass, occurring in regular crystals and irregular grains, and ap- 
parently the first formed constituent of the rock. Hypersthene 
is next in amount, of a rather pale color as compared with that 
in the Cortland peridotites, and often full of minute inclusions. 
It is seldom or never at all idiomorphic, and usually tends to 
collect around the olivine. A nearly colorless monoclinic pyr- 
oxene is almost equally abundant, with habit like that of the 
hypersthene. It does not show any diallagic cleavage; this 
character, however, is found in the pyroxene of the small iso- 
lated exposure half-way between Indiantown and Dolin’s Lake. 
Triclinic feldspar is seldom entirely absent from a slide, but 
is sometimes quite subordinate. The remaining constituents of 
any importance are spinel and magnetite. The former is a very 
abundant accessory, occuring in irregular grains and minute 
vermiform shapes, and is probably largely secondary. Magnet- 
ite, too, appears to be chiefly an alteration product occurring in 
minute granules and irregular aggregates; but a few larger 
grains may be seen in unaltered parts of the slide. Apatite is 
very rarely seen. 
REACTION RIMS. <A very marked character in the thin sections 
is the invariable presence of reaction rims, surrounding the 
olivine crystals where they come in contact with the feldspar. 
These are composed of two zones: 
1. The inner zone is very narrow, composed of a series of 
small highly refracting grains with green and pink pleochroism, 
and is continuous with the larger grains of hypersthene. 
2. A zone of fine radiating needles of a faint green color and 
pleochroism which seem to be uralitic amphibole. The outer 
part of this zone is filled with minute vermicular grains of a_ 
very high refracting, deep green non-pleochroic mineral, prob- 
* U.S. G.S. Bull. 28, p. 27. 
