144 PROCEEDINGS; BOSTON SOCIETY NATURAL HISTORY. 
Dikes newer than the granite. A large diabase dike occurs on 
Mt. Pleasant, containing a handsome rose-colored augite that appears 
like garnet except for its low refraction. The rock is much chlori- 
tized, the chlorite extending all through the feldspar. Many 
minute quartz veins ramify through the rock. 
An especially fresh and handsome dike rock (PI. 2 , Fig. 8), form¬ 
ing a dike 35 feet wide, occurs on Pine Hill. It shows strikingly 
the opliitic texture. The plagioclases are in lath-shaped forms, 
sometimes fibrous, and of a greenish color. The interspaces are 
filled by masses of a handsome pink augite. Iron ore is abundant, 
chiefly in grains of irregular outline, hence presumably ilmenite. 
Many small needles of some mineral not definitely determined are 
visible without polarized light. It approaches the augite porphyrite 
of Predotzo in appearance. 
A dike five and one half feet wide, of dark color, and similar to 
the above dikes, cuts the granite on a private way leading south 
from Broad Street, near Vine Street, in Weymouth. It is rich in 
pyrite and shows considerable effects of crushing. A dike from fifty 
to sixty feet wide runs between the granite and slate and the con¬ 
glomerate on the west side of North Common Street. The dike 
runs with the stratification of the slate and is almost a gabbro. A 
pink decomposition product occurs throughout it. 
GRANITE DIKES. 
A granite dike three inches wide cuts both the slates and the 
primary dikes in the localhy east of Randolph Avenue, near Pine 
Tree Brook already referred to. The microscope shows it to be of 
decayed minute grains with granophyric texture, and pegmatitic 
growths interpenetrating the feldspar and quartz. (Plate 5 , Fig. 14 .) 
Another granite dike, similar in character, was noted between 
Common Street and the North Common Hill quarries, not far from 
to sixty feet wide. 
the corner of Adams Street. The dike is from fifty 
CONTACT PHENOMENA. 
The contacts between the granite and slate are remarkably dis¬ 
tinct, and even in the thinnest sections the sharply defined line of 
contact is evident. The dark particles of the slate tend to collect 
