WHITE : PETROGRAPHY OF THE BOSTON BASIN. 143 
ilmenite occurs in elongated fragments, and is slightly altered along 
the edges to the white decomposition product, leucoxene. Garnets 
of light color occur scattered through the rock, together with a very 
little quartz. 
A dike 15 feet wide occurs on the side of Eldridge Hill. It is of 
similar rock, but more decomposed, the ground mass being chiefly 
converted into an indefinite brown or black mass of specks with 
abundant and only slightly altered ilmenite in long shreds and irreg¬ 
ular aggregates, and with considerable of the chloritic alteration 
product. 
A dike two and a half feet wide cuts the slate at Mill Cove. It 
is of a dark green color, with darker markings, which present an 
oily appearance to the naked eye. Its microscopic characters are 
so altered as to make specialized determination impossible. This is 
also the case with the two dikes near by at Mill Cove Dam. 
An extensive dike 300 feet wide cuts the slate on Quincy Avenue, 
opposite the brick store, adjacent to a similar small dike. The 
ground mass is composed of dirty brown alteration masses, in which 
are imbedded lath-shaped feldspars which give an extinction angle 
of about 7°. Bending of the crystals and flow structure are observ¬ 
able. Triangular magnetite crystals are present, and a little ilmenite 
somewhat altered to leucoxene. 
A large primary dike occurs south of Pine Tree Brook in the 
Blue Hill Reservation. It seems to be a strongly gabbroitic devel¬ 
opment of the diabase, containing in portions conspicuous shreds of 
feldspar. The augite passes into hornblende on the margins, with a 
central core of unaltered augite or pyroxene. Magnetite crystals 
are abundant. 
Dikes older than the granite. Near the target of the shooting 
range in the Blue Hill Reservation is a large dike, cut by and hence 
older than the granite. It proved in section to be a coarse diabase 
with uralitized green hornblende, resulting from chloritic decomposi¬ 
tion, and a fresher leek-green hornblende, twinned feldspars with ten¬ 
dency to opliitic texture, and small secondary feldspar rods. Some 
magnetite was also present. 
A similar primary dike, also in the Reservation, occurs east of Pine 
Tree Brook, near Walnut Ave. This is also a diabase, but of very 
fine-grained ground mass, and with striking rod-shaped crystals. 
The chlorite is slight in amount, but considerable magnetite is 
present. 
