98 
of Granite Plateau. The faet that we cannot trace this dike 
above the basal conglomerate is thus readily explained ; for this 
conglomerate was then the surface deposit, the newest sediment 
upon the ocean floor. "Taking all these features into consider- 
ation, it is doubtful if there can be found elsewhere in the Bos- 
ton Dasin во clear an exhibition of a complete eruption—com- 
plete in both its intrusive and extrusive phases. 
Correlation of the Central and Western .Areas.— 
Up to this point the stratigraphy of the western area presents 
no serious or insuperable difficulty, each group of ledges pre- 
senting certain recognizable features common to some of the 
others. But on attempting to extend this correlation to the 
central area the case is very different; for the break between 
Great and Crescent Hills, the only point where the two areas are 
in contact, is so profound that at first glance the two sections 
seem to have nothing in common. The peculiar first melaphyr 
and the porphyrite, which are the principal keys to the western 
area, are certainly wholly wanting in the central area. I have 
already stated that the heavy bed of conglomerate forming Con- 
glomerate Plateau, Round Hill, and, probably, the ledges to 
the eastward as far as Conglomerate Island and Green Hill, 
should, apparently, be correlated with the conglomerate under- 
lying the lower amygdaloidal melaphyr at the northern end of 
the section along the railroad. If this is done, then, while the 
western area consists, exclusive of the fundamental granite and 
Melaphyr Peninsula, of three beds of conglomerate and two in- 
tercalated flows of lava —one basic and one acid ; the central area 
is made up of two beds of conglomerate, each of which is over- 
lain by a flow of amygdaloidal melaphyr. The base of the lower 
and prineipal conglomerate is nowhere exposed ; but I see no 
reason to doubt that it rests upon the porphyrite, and is the 
equivalent of the third conglomerate in the western area. This 
interpretation of the facts has the merit of simplicity, and it 
leaves no observed facts unexplained. The summit of the 
