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15 
headland opposite Brush Island. Here we pass very abruptly 
from the coarse, massive granite to the mixed granite and diorite, 
for the first hundred feet or so a very confused mixture, in which 
the diorite seems to predominate; but west of that, along the 
entire stretch of shore, as far as Green Hill and Nantasket, the 
granite is the prevailing rock, with frequent inclusions of diorite. 
This is, structurally, an extremely interesting section, and it will 
be deseribed more fully a little farther on. The essentially 
patehy distribution of the diorite in the granite makes the tra- 
eing of this rock in the weathered and lichen-covered inland 
ledges rather unsatisfactory, not to say unprofitable, and only 
enough work has been done in this direction to show that what 
we can see so clearly along the shore is really characteristic of 
the whole town. Thus it must be evident to any one observing 
the ledges along the west side of Little Harbor and on Forest 
Avenue that the mixed granite and diorite extends inland a con- 
siderable distance; while in other parts of the town it is equally 
clear that coarse and massive granite covers large areas. 
The diorite is always dark-colored and holocrystalline, but 
usually rather fine-grained, varying in texture from compact or 
aphanitic to distinctly but not coarsely crystalline, z. e., the dio- 
rite is rarely coarse in the sense that the granite often is. On the 
other hand, it rarely resembles diabase, except in the most com- 
pact forms, the normal difference in crystalline structure or 
habit being readily recognized in the macrocrystalline diorite. 
Under the miscrocope it is usually seen to be composed chiefly 
of plagioclase and hornblende. The feldspar is commonly 
rather opaque ; but the hornblende is, in many cases, beauti- 
fully clear and dichroic, although usually bordered by secondary 
biotite, chlorite, ete. ; while in the more compact and highly 
altered diorite the hornblendic element is very largely reduced 
to hydrous silicates, which give the rock a dark greenish color. 
Among the secondary minerals occurring in veinlets and irregu- 
lar segregations epidote is most prominent, but it is frequently 
accompanied by chlorite and quartz. The black oxides of 
iron (magnetite and menaccanite ) are usually present but rarely 
