48 
mingled indiscriminately, and, with the exception of a rare 
pebble of felsite or granite, the entire ledge, sandstone as well 
as conglomerate, appears at first to have been derived from the 
Long Beach Rock flow of melaphyr. There is much, however, 
in both the composition and texture of this mass to suggest that 
it may be a true volcanic tuff, a record of explosive volcanic 
action either at the close of the eruption already described or 
at the inception of the similar littoral or submarine eruption by 
which this bed was covered. It will be observed that this in- ` 
terpretation is adopted on the map, the lava-flows being thus 
supposed to have followed each other so closely over this 
marine area as to preclude the deposition of any appreciable 
amount of the ordinary or normal sediments between them. 
This bed of tuff strikes N. 65? E., and dips S.E. 15°. Its 
full breadth appears to be exposed, viz., 120 feet, equal to a 
thickness of about 30 feet. It can be traced for about 300 feet 
along the base of Atlantie Hill, rising at the highest point some 
20 feet above the beach. The line of strike carries it directly 
across the shingle beach connecting Long Beach Rock and At- 
lantic Hill. I have been unable to positively identify it on the 
low-tide ledge north of Centre Hill; but it seems impossible to 
doubt that it crosses here, although probably with diminished 
thickness. It appears necessary, also, to suppose that the tufl 
overlies conformably the melaphyr of Long Beach Rock, al- 
though there is no exposure of the contact. If the tuff extends 
so far to the eastward, it must pass to the north of Gun Rock 
and between the two Black Rock islets. 
This old ash-bed is overlain clearly and conformably by a 
second bed of mélaphyr, the contact being well exposed at sev- 
eral points along the northern base of Atlantic Hill. "The con- 
tact is similar to that between the green slate and overlying 
melaphyr on. Long Beach Rock — conformable in the general 
view and yet with many minor irregularities. Fig. 2 shows 
the contact as it appears at the edge of the beach, below the 
bath house and near the foot of the Atlantic House steps. It 
is clearly such a contact as would naturally result from the 
