9T 
granules, and with little if any truly amorphous base. Im- 
bedded in this feldspathic ground-mass are numerous large, clear 
and often greatly corroded plagioclases. A few small apatites 
are present. The section shows a well-marked flow-structure. 
Chemical analysis yielded 58.25 per cent. of silica ; and an aver- 
age of four determinations gave a specifie gravity of 2.78. 
THe DIKES OR INTRUSIVE Rocks. 
The dikes, as has been noted by Professor Crosby, are all of 
normal form — sharply defined, wall-like masses making high 
angles with the horizon and invariably cutting across the strat- 
ified rocks ; no intrusive beds or sheets having been observed in 
the Nantasket district, owing, probably, to the absence of thin 
bedded or slaty sediments. As previously stated, the dikes 
consist mainly of араке ; but they also include an important 
series of porphyrite dikes and at least one large dike of mela- 
phyr. The dikes of melaphyr and porphyrite are believed by Pro- 
fessor Crosby to be contemporancous with the surface flows 
of those rocks ; while he finds that the numerous diabase dikes 
are certainly newer in most, and probably in all, cases than the 
melaphyr and porphyrite. Hence, following the chronological 
order, we begin with the last named rocks. 
The Melaphyr Dikes. 
The dike of melaphyr (1) on the north side of Centre Hill is 
80 evidently identical with the melaphyr which it intersects that 
it was not examined microscopically. The large dike (2) in 
the western area, which Professor Crosby regards as contem- 
poraneous with the first flow of melaphyr, is, like that flow, of 
doubtful character. Macroscopically, it bears a marked resem- 
blance to this flow, and is contrasted in appearanee with all the 
diabase dikes of the district. Under the microscope it is seen 
to be a fresher and more crystalline rock than the melaphyr, 
showing in the section a ground-mass of feldspar microlites and 
