1893.] 
501 
[Crosby. 
ate series to the great slate series. It appears probable that, in a 
general way, the Hingham ledges supplement the Xantasket 
ledges, the basal beds of conglomerate having a remarkably fine 
development in the latter, while the former afford continuous 
exposures of the upper beds of conglomerate and the overlying 
slate. 
THE GRANITIC ROCKS. 
The relations of the diorite to the true granite indicate that it 
is in every case clearly the older rock. It is everywhere inter- 
sected by numerous, irregular, branching dikes of granite ; and 
the granite ledges are rarely quite free from angular inclusions of 
diorite. Although the relations of the diorite to the granite are 
so intimate that its outlines do not admit of accurate definition, it 
has been found to occur abundantly only in a limited, irregular, 
and interrupted east- west belt near the northern edge of the 
granite, the best exposures being on or near Hull Street, Weir 
River Lane, Kilby and East Streets, and on Fort Hill and the 
adjacent ledges. The diorite nowhere exhibits a distinct flow- 
structure ; but it is usually quite massive, finely crystalline, and 
dark-colored. Occasionally, however, it is coarser, with the 
hornblende either very clearly and prominently developed, or 
mainly wanting, giving a light-colored, feldspathic variety. Epi- 
dote is, as usual, the most conspicuous secondary mineral, oc- 
curring chiefly as narrow and irregular segregations and veinlets, 
especially along the joint-cracks. 
By far the greater part of the granite of Hingham belongs to 
the sparingly hornblendic, usually coarsely and distinctly crystal- 
line. gray to pink or red variety of the South Shore district. The 
hornblendic element is very generally replaced partially, some- 
times wholly, by mica (chiefly biotite) . Irregular dikes of the 
more finely crystalline or micro-crystalline granite, and of felsite, 
are frequently observed cutting through the coarser granites and 
also the diorite. 
The felsite of Hingham is not wholly intrusive or in the form 
of dikes. The gray felsite on the north side of Beal Street, at 
the western end of the granite (PI. XY) is quite probably part of a 
surface flow ; and the beautiful red felsite occurring so plentifully 
in the form of boulders, in the vicinity of Thaxter and Lincoln 
Streets, is unquestionably effusive. The Beal Street felsite encloses 
