16 
abundant, and the rock also often contains a small amount of 
original quartz. The quartz diorite is especially interesting as L 
indieating a gradual passage to the more basic granites. 4 
The diorite is undoubtedly a plutonic rock ; and it is sufficient- 
ly varied in character to suggest that it is possibly not all of the | 
same age; but no facts have been observed which point to a | 
definite conclusion, and all that can be regarded as well deter- | 
mined is that its relations to the granite are always essentially 
the same. 
The granite, on the other hand, belongs very clearly to two, 
and probably three, more or less distinet periods of igneous 
activity, or successive phases of the same period. First in order 
of time comes the granite which is most intimately associated 
with the diorite. This is very abundant, and partakes of the 
character of the diorite. It is on the average only a little more 
coarsely crystalline than the diorite; and usually contains suffi- 
cient hornblende or black mica to make it quite dark colored 
for a granite. The quartz is often deficient, and the feld- | 
spar is partly plagioclase, so that it would be easy to mistake | 
a portion of the rock for either diorite or syenite. The horn- 
blende shows, perhaps, even more alteration than in the diorite, 
being very largely replaced by chlorite. Next in order comes 
the light gray and pinkish granite, which is usually coarsely 
. crystalline and massive and is as a rule comparatively free from 
inclusions of diorite. This rock is rich in acid feldspar and 
quartz ; but the hornblendic element is usually quite scantily de- 
veloped ; and black mica (biotite) often partially, sometimes 
wholly, replaces the hornblende, while both of these accessories 
are, as arule, largely altered to chlorite or other hydrous species. 
It can be seen breaking through the first granite and the diorite at 
many points along the shore, especially between Little Harbor 
and Nantasket ; and some of the smaller dikes of this granite are 
quite fine grained and not easily distinguished from the third | 
type. This coarse acid granite is the only rock in this district 
of any particular economie interest ; and it has been quarried 
only to a very limited extent. There is one small quarry on the 
