56 
phyr described up to this point, with the included tuffs, on 
Rockland, Atlantie, and Centre Hills, Gun hock, the sub- 
merged ledges and Little Black Rock, belongs to one great bed 
or sheet having an approximate ly east-west trend and southe ly 
dip and overlying the conglomerate forming the north end of 
Long Beach Rock. Throughout this great mass, however, 
there are many indications that it is re ally composite, consisting 
of a succession of flows; which either were not submarine, or 
followed each other so r: ipidly ав to preclude the formation of 
any sensible thickness of intervening sediments. The included 
beds of tuff show this very clearly. The zones of brecciated 
melaphyr, the fractures being marked by segrations of silica 
and impure epidote, seem to show it. Апа there are often 
curving lines, such as have been described at the base of the 
second tuff (Fig. 4), which appear to mark the contact of two 
successive flows. 
On the north side of Centre Hill, near the end of Centre 
Hill Avenue, is another proof, in the form of a dike of mela- 
phyr (No. 1, on the map), that this great bed is not 
throughout of the same age. Its compactness and general 
resemblance to the enclosing melaphyr, as well as the irreg- 
ular contacts, mark this dike as prob: ibly contemporaneous with 
some later flow of this immediate series, although, of course 
of later date than the flow which it is seen to intersect. 
The general absence of the amygdaloidal texture in this mel- 
aphyr, except in the flow separating the second and third tuffs, 
is very noticeable; and this negative character, as well as the 
prevalent brecciation, contrasts it strongly with the most of the 
other masses of melaphyr or basic lava in the Nantasket area. 
In fact, as Mr. Merrill's descriptions show, both the structure 
and composition of this rock assign it a position between the 
ultra basie lava or typical melaphyr, and the more acid lava or 
porphyrite of this region. 
Porphyrite.—The remaining rocks and islets, east of Gun 
Rock, constitute a very typical area of porphyrite, bounded on 
