MUSEUM OF COMPARATIVE ZOOLOGY. 3 
No outcrops of the coarse rock have been found south of the Granite 
Street quarry. Professor Crosby has included the fine-grained diabase 
which crops out at the Pumping Station in Brighton, and similar rocks 
in Brookline and Newton, as a part of this dike ; but the great change 
of strike required, and the long intervening distance without exposures, 
are opposed to the supposition. From the Granite Street quarries to 
the Old Powder House in Somerville, (a distance of about one and a half 
miles,) the strike of the exposures is N. 25° W. From that point to 
Spot Pond, the trend is N. 10° E. In Medford and Somerville the 
country rock is argillite, which has been thrown into gentle folds, the 
dips of which seldom exceed 35°. A notable exception to this state- 
ment is seen at the old slate quarry on Professor’s Row, College Hill, 
where beds strike +N. 95° E. and dip +72° to the south. ‘The area 
of coarse diabase, which has an average width of about two thousand feet, 
is never found in contact with the slate. The exposures of diabase al- 
most invariably show the well-known weathering to boulders in situ, 
though this is best observed at Pine Hill. North of High Street in 
Medford the areal geology is complicated by the occurrence of granite 
and felsite, for the mapping of which very detailed field-work will be 
necessary. 
The arrangement of exposures of coarse and fine grained diabase in 
the vicinity of the Old Powder House seems to show a gradual passing 
of one rock into the other. In the immediate vicinity of the Powder 
House is an extensive outcrop of coarse rock, like that at the Granite 
Street quarries and Pine Hill. About four hundred feet northeast of the 
Powder House on Harvard Street the texture is much finer, though not 
sufficiently fine to be ranked with the normal “ greenstone.” About six 
hundred feet S. 20° W. of the Powder House the rock is somewhat finer 
than at the last-mentioned locality. Again, at the corner of Elm and 
Morrison Streets, which is about one thousand feet west-southwest of the 
Powder House, the normal “greenstone” occurs in slate. Moreover, on 
Willow Avenue, about fifteen hundred feet along the strike to the south 
from the Harvard Street locality, the rock is practically identical with that 
_at the latter place. From this it seems probable that the coarsely crystal- 
line rock at the Powder House is near the middle of the dike, where the 
cooling was slow, and that the gradual diminution in the size of the 
grains in going from that point is owing to more rapid cooling near 
the contact. 
The wide distribution of the “greenstone” has made it impracticable 
for me to make a complete examination of it, but the few localities which 
