57 
the north, apparently, by the eastward extension of the great 
belt of melaphyr indicated by Little Black Rock, and on the 
south by the conglomerate of Green Hill. Although, as Mr. 
Merrill states, this rock resembles the melaphyrs more in its 
general aspect than does the porphyrite of the western part of 
the Nantasket area ; chemical analysis shows that it is as acid 
as any porphyrite in this district. It is somewhat variable in 
its microscopic features, but usually presents a compact, or 
apparently felsitic, dark gray or purplish to nearly black, base, 
enclosing numerous porphyritically developed feldspars or 
minute aggregates of epidote. It is never amygdaloidal, and 
‘arely distinctly breeciated ; but exhibits at several points а 
well-developed striping or flow-structure. 
Black Rock, which is about seven hundred feet east and west, 
and half as broad, is a continuous and almost perfectly bare mass 
of porphyrite. Fluidal lines are very plainly marked in a por- 
tion of the porphyrite, with usually an approximately east-west 
trend and a gentle southerly dip, thus proving that the normal 
attitude of the rocks, so clearly exhibited about Atlantie Hill, 
is preserved to the very eastern limit of the Nantasket area. 
Near the middle of the west half of the island, a considerable 
mass of porphyrite is split up by close, parallel east-west and 
nearly vertical joints so as to present a very shaly appearance. 
The character and especially the attitude, or dip and strike, of 
this structure indicates that it is possibly true cleavage rather 
than jointing, and analogous to the foliation of the diabase on 
Calf Island.! The small island north of Green Hill (Green Hill 
Rock) is another mass of gray and purplish porphyrite, with flu- 
idal lines ; and the half-tide ledge between this and Black Rock 
exhibits a similar constitution. Since the porphyrite of this 
area is exposed only in these three insular masses, it is mani- 
festly impossible to determine with certainty either its bounda- 
ries, its thickness or its relations to the bordering melaphyr and 
conglomerate. If, however, we may follow the lithologic indica- 
tions and correlate it with the porphyrite of the western area, it 
must, as will appear later, be referred to a horizon below this 
“POG, B; Sv N. H. xxi. 455, 
