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103 
The porphyrite is, possibly, not wholly wanting on Rocky 
Neck ; for the small point jutting into Weir River Day south of 
the basal conglomerate and mapped as granite, consiste of angu- 
lar masses, probably overlying a ledge, of a semi-erystalline, red 
rock, apparently quartzless, and resembling the Nantasket por- 
phyrite. Its rather crystalline texture would indicate a dike of 
porphyrite breaking through the granite ; but if it is effusive, a 
surface flow, important bounding faults must be assumed to ac- 
count for its topographic position with reference to the granite. 
The Nantasket Peninsula North of Atlantic Hill. 
As previously stated, the only exposure of the underlying 
rocks north of Atlantic Hill is the ledge of slate on the shore, 
south of Thornbush Hill; and with the exception of thé ledge 
of green and gray contorted slate on the railroad northwest of 
Rockland Hill, which has been doubtfully referred to the prin- 
cipal bed of tuff in the Atlantic Hill section, this is the only 
considerable outcrop of slate on the Nantasket Peninsula. It 
is exposed along the shore for about 900 feet and with an ex- 
treme breadth across the strike of perhaps 400 feet. It is a 
‘ather coarse or arenaceous, purplish slate, distinctly and finely 
banded with gray, and occasionally with greenish, layers. The 
strike is N. 65°-70° E. ; and the dip is variable, but always low. 
In the western ledges it is S. 10°-15° or 20° at the most; but 
in the middle ledges the beds are approximately horizontal, 
broad contortions or undulations showing gentle dips in various 
directions ; while the eastern ledges show a northerly dip of 
10°-20°. This is evidently an oblique section of a flat anti- 
cline. It includes no pebbly or distinctly arenaceous beds; 
and the exposed thickness of the slate, although difficult to es- 
timate, probably does not exceed 40 or 50 feet. The slate is 
characterized throughout by a distinct cleavage-structure, which 
strikes N. 80° E., and hades, usually to the north, occasionally 
to the south, 0°-5°, being always transverse to the bedding 
