WHITE: PETROGRAPHY OF THE BOSTON BASIN. 
129 
orthoclase crystals often attain a length of 12 mm. and predominate, 
their abundance constituting the leading character of the rock ; while 
the proportion of quartz varies, the dark silicates being least abun¬ 
dant. According to Merrill (’89, p. 409 ; also ’84, p. 10), the Quincy 
granite also contains a very brittle variety of pyroxene, in addition 
to the hornblende, which tends to break out as the stone weathers 
so that a polished surface in time becomes pitted. This is not, 
however, uniformly the case with the stone now quarried. In 
many of the sections examined no pyroxene was present. Dr. 
Merrill has kindly permitted me to examine the original slides pre¬ 
pared for the Tenth Census and now in the U. S. National Museum. 
One fragment in these sections, at least, seems to be pyroxene, while 
there are various dark brownish and greenish associated minerals, 
which Dr. Hawes, who originally determined the slides, regarded as 
products of alteration of the pyroxene. In other slides examined, 
however, unaltered pyroxene occurs. 
Microscopically the rock presents a typically granitic texture, 
without differentiation of ground mass and phenocrysts. The feld¬ 
spar is practically all orthoclase with possibly a little plagioclase. It 
is for the most part simply twinned, according to the Carlsbad law, and 
is little altered. Microperthitic texture due to intergrowth of ortho¬ 
clase and albite is prominent, and often takes place in a striking 
manner at an angle of 65J degrees with the cleavages. It sometimes 
also assumes a “shoe-peg texture ” like that often seen in melilite. 
Occasional inclusions of minute crystals of indeterminable minerals 
are noticeable in the feldspar. They are probably of hornblende. 
The fluid inclusions noted by Wadsworth (’78, p. 310) in the Rock- 
port granites, which are in many respects similar, are not apparent. 
Rarely lines of weakness are noticeable, which develop the “rift” 
of the rock (Tarr, ’ 91, p. 267). 
Amphibole is much the more abundant dark silicate. It is usually 
a hornblende, apparently of a rich soda variety, as the analyses of the 
rock also indicate, in densely opaque masses. On the edges, how¬ 
ever, these masses are, in the thinnest sections, seen to be composed 
of many fine rod-like needles felted together and fringed with pro¬ 
jecting needles. Often the center of the mass does not seem to be 
so high in soda as the edges. A similar case has been figured-by 
Cross (’90, p. 359) in the case of certain pyroxenic growths. The 
needles are also seen in bundles or rosettes of blue or black acicular 
crystals among the feldspars, arranged in a manner similar to the 
