4T 
rests should be regarded as an exceedingly fine volcanic tuff, 
the eruption of liquid lava having been preceded by an out- 
burst of volcanic ashes. The influence of the lava flow has also 
penetrated the conglomerate, the layer of sandstone eight feet 
below being almost as thoroughly indurated as the slate. 
The breadth of the melaphyr is fully two hundred feet and its 
actual thickness probably not far from sixty feet. It is, for the 
most part, rather compact and of a dark green color. But 
toward the top especially it is full of irregular segregations of 
quartz and shows, in a rather large way, a very distinct and 
somewhat wavy flow-structure, resembling bedding and parallel 
With the stratification of the underlying conglomerate. Irreg— 
ular veins and masses of compact and impure epidote, occa- 
Sionally of considerable size, also occur in the upper part of the 
melaphyr. It is very plain that the entire thickness of mela- 
phyr is to be regarded as forming one simple and normal flow 
homogeneous and massive below, but superficially brecciated 
and scoriaceous. 
This great lava-flow is undoubtedly continued eastward in the 
outermost part of the ledge exposed at low tide north of Centre 
Hill ; and it appears probable that further east the line of strike 
changes 80 as to connect Little Black Rock with the same bed. 
This islet, whieh is about four hundred feet long and half as 
broad, is entirely composed of melaphyr very similar to that on 
Long Beach Rock, except that it is in part amygdaloidal. 
The prominent ledge at the northwestern base of Atlantic 
Hill, and partially isolated by the sands of Nantasket Beach, is 
Chiefly a dict; | 
hiefly ü distinetly and evenly bedded greenish gray sandstone, 
alternatin g 
especially in the upper part, with layers of greenish 
and т; 
ег indistinct, small-pebbled conglomerate. This con- 
[ni A: . i MISSA 
glomerate is made up of much smaller pebbles, and is in every 
WAY Very dict! . F 
м Very distinct from that on Long Beach Rock. The peb- 
)les ar Yat > ] 
es are mainly more or less angular fragments of melaphyr, 
аге : х ү " 
ne not well assorted, the coarsest and finest material being 
© 
