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19 
diorite also possessed a gneissic structure of some sort before the 
eruption of the granite ; in fact this can frequently be observed 
in the more lenticular masses of diorite. The diorite, then, has 
been injected by the granite chiefly along its own original struct- 
ure-planes ; and the flow-structure of the granite is probably due 
largely to its having come up between these parallel walls of 
diorite. 
Breaking through this gneissoid granite and the enclosed dio- 
rite at irregular intervals are many small and some large masses 
of the coarse, light-colored granite. These usually conform in 
direction, at least approximately, with the flow-structure of the 
older granite, and exhibit in this direction a similar but, as a 
rule, less distinct flow-structure. We thus not only have a 
somewhat eneissoid diorite of probable igneous origin broken 
through very profusely by, and enclosed in, a highly perfeet 
gneissoid granite — a true eruptive rock which, but for its rela- 
tions to the diorite, might readily be classed as of sedimentary 
origin, a true gneiss; but both of these terranes are traversed 
again and again in a way possible only to an igneous rock, by 
à second and more acid pseudo-eneiss. Again, all the preceding 
rocks are injected by occasional small and irregular dikes of micro- 
granite and felsite, which, for the most part, are devoid of 
gneissic structure. And, finally, this entire sequence of granitic 
eruptions is divided by a well-defined series of porphyrite dikes 
and no fewer than three distinct systems of diabase dikes : all 
of which will be described in connection with the dikes of Nan- 
tasket. 
This shore undoubtedly presents the best general section of 
the plutonic rocks of the Boston Basin. Certainly at no other 
point do we find plutonic masses of so many different ages so 
clearly exposed in their normal relations ; and in this connection 
it should be noted that the passage (in natural sequence) from 
the diorite through the more basic to the more acid granite 
affords some indication that the diorite is not widely separated 
in time from the granites, the entire series constituting but one 
complete igneous cycle. 
