134 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
It is of a fine-grained texture, and composed largely of white or 
pink feldspar, usually of fine rectangularly crystalline form, with 
scattered grains of dark silicate and very little quartz. In North 
Weymouth, and in Quincy, west of the railroad between West 
Quincy and East Milton, the rock is considerably decomposed and 
the dark silicates are more in evidence. Similar granites occur along 
Long Island Sound in the vicinity of Niantic and New London, 
Conn., and also at Laurium in Greece. 
Unlike the biotitic type of granite previously described, this granite 
shows no plagioclase, but the numerous rectangularly idiomorphic 
grains are of monoclinic feldspar. This feldspar is shown in most 
instances to be shot through with innumerable acicular crystals of 
the blue hornblende already described in the normal hornblendic 
granite, to which this granite type is so closely related. The 
needles gathered into compact clusters also form masses by them¬ 
selves among the other minerals. The biotite in small brown flakes 
is quite abundantly scattered through the rock, often lying directly 
adjacent to the hornblende masses. The feldspar shows no 
twinning. As already stated the quartz is present only in small 
quantity, and is of the smoky variety. No accessory minerals 
were observed. 
The rock is, then, a hornblende-biotite-granite, or, to be briefer, a 
hornblende granitite. 
PORPHYRITIC AND FELSITIC ROCES. 
Porphyritic Aporhyolite. — Where the latter type of horn- 
blendic-granitite is lacking, the normal (Quincy) hornblendic granite, 
passes, without visible change in the field, into a rock which under 
the microscope exhibits a microgranitic ground mass without 
granophyric texture, composed of a finely crystalline aggregate of 
monoclinic feldspar, quartz, and unaltered dark green or ultra¬ 
marine-blue hornblende. The latter makes up about one fourth the 
mass of the rock. A few grains of hematite are also present. 
Occasional larger grains of feldspar exhibit a microperthitic 
texture. 
The phenocrysts consist of a monoclinic feldspar and quartz, the 
former predominating. The feldspars vary up to 5 or 6 mm. in length, 
and are very regular in outline, the faces iP, and AT being chiefly 
developed. Twinning according to the Carlsbad law is conspicuous. 
