18 
granite, should, on account of their resemblance to it in com- 
position, be regarded as, in a general view, essentially syn- 
chronous. 
This coast abounds in instructive exposures of the granitic 
rocks, but the nearly continuous belt of wave-washed ledges 
along the Jerusalem Road between Nantasket and Beach Island 
is particularly worthy of thorough study, illustrating, as it does, 
the relations of all the rocks and presenting some features which 
are not clearly exposed elsewhere. On the south side of Besch 
Island, as already noted, there is a fine development of the 
coarsely crystalline, massive granite ; but from the junction of 
this with the mixed granite and diorite just south of the artificial 
harbor to Nantasket the finer-grained, dark-colored, older granite 
is the principal, and for a considerable portion of the distance, 
excepting the dikes, almost the only, rock. This older granite 
is characterized throughout the entire section by a remarkable 
gneissoid structure due to the flowing of the material while in a 
viscous condition. At many points this flow-structure is so 
perfectly developed that the rock presents a distinctly gneissic 
character even in small hand-specimens ; and where the fluidal 
lines are least obvious, they are still traceable on the broad, 
clean surfaces of the ledges. The trend of the flow-structure or 
pseudo-stratification varies somewhat. It is about N. IZ. -S. W. 
on Beach Island; while farther west it ranges usually between 
N.-S. and N. W.—S.E., but becomes N. 60° W. as we approach 
the beach leading to Green Hill. The dip is usually nearly 
vertical, although sometimes as low as 60? or less. 
This gneissoid granite encloses at most points numerous frag- 
ments of diorite of varying lithological character. These exhibit 
a great range in size; and they are usually more or less elongated 
in form, being often distinctly lenticular and indicating in their 
smooth and somewhat indefinite outlines a partial fusion of the 
diorite in the melted granite. The elongated fragments coincide 
in direction with the flow-structure of the granite, and this struct- 
ure is always most perfect where the diorite is most abundant. 
The form of the diorite fragments means, probably, that the 
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