2o6 The American Geologist. April, i8t)>v 
rarely, distinct crystals of quartz. * * * The sienitic 
greenstone" (pegmatyte) "is crossed in various directions by 
fissures, the walls of some of which are encrusted with thin 
layers of feldspar: others are filled up with this mineral."* 
These fine veins of microcline and quartz are abundant on 
the west wall of the Granite street quarry near its northwest 
corner, where occurs also a large mass about six feet in width 
of coarse drusy rock, showing long feldspar laths an inch 
or more in length, usually with a triangular arrangement, the 
interspaces being filled with quartz and chloritic substance. 
The contact of this mass with the finer grained diabase is 
quite sharply marked in sinuous curves; in the diabase dark 
mica and pink feldspar are very abundantly developed near 
the contact. This mass seems to have been a more coarsely 
crystalline gabbroid segregation in the original diabase, in 
which the coarser quality of the miarolytic pores permitted in- 
filtration of the pegmatyte-forming fluids more freely. The 
drusy cavities now developed by weathering are frequently 
from one to two inches in length, and contain crystals of 
microcline feldspar, quartz and amphibole. 
Similar drusy openings are found in the pegmatyte veins 
which penetrate the normal diabase; and the walls of these 
veins are not sharply marked, for we find the feldspars of the 
diabase kaolinized and salmon-colored in the zone next to the 
pegmatyte, showing by their wavy extinctions in thin section 
an acid border on the side next to the triangular interspaces. 
Still nearer to the vein the ophitic structure of the diabase 
gives place to the development of plump squarish microcline 
crystals irregularly distributed, with occasional quartz. A 
series of three thin sections, made from a single specimen of 
the rock at distances of lo cm., 4 cm., and o cm. from a quartz- 
microcline druse, shows the transition from the normal dia- 
base structure through the "quartz-diabase" phase to a normal 
vein pegmatyte. In some cases the "quartz-diabase" zone is 
much wider, the rock showing for several feet a considerable 
amount of interstitial quartz, always, however, accompanied 
by more or less of the salmon-colored feldspar. In these lo- 
calities there is no immediate evidence, in either the hand 
*Boston Journal of Philosophy and Arts, 1825, vol. II, p. 277; and 
vol. Ill, p. 486. 
