The structure of other districts is, no doubt, equally varied 
and intricate ; but Nantasket excels in the very favorable op- 
portunities which its almost continuous rock surfaces present 
for deciphering its structure lines. t is especially interesting, 
in this connection, to observe that, while the rocky area occur- 
ring next to the north of Nantasket, in the middle line or axis 
of the Boston Basin—the outer islands of Boston Harbor— 
affords the finest and most typical exposures of intrusive sheets 
or interbedded dikes to be found in this region; the characters 
of the contemporaneous beds or ancient lava flows of the Boston 
Basin and their relations to the interstratified conglomerate can 
be studied to the best advantage at Nantasket, on the border of 
the basin. 
At no other point is the evidence equally clear and conclusive 
that the melaphyrs and porphyrites of the Boston Basin are, in 
the main, true contemporaneous lavas which were poured out 
on the sea-floor at different periods during the deposition of the 
beds of conglomerate and sandstone. The Nantasket area also 
shows more clearly than any other that in some parts, at least, 
of the Boston Basin the conglomerate beds, as well as the as- 
sociated melaphyrs rest directly upon the fundamental granite, 
without the possible interposition of the slate series. 
It may be pointed out in this conneetion that the main pur- 
pose of the present investigation is simply to elucidate the 
structure and explain the origin and sequence of the rocks 
of this somewhat eomplicated area. In other words, this is a 
structural rather than a lithological study; and I wish to ac- 
reat obligation to Mr. Geo. P. Merrill of the 
O1 
= 
U. S. National Museum for undertaking a microscopic examina- 
knowledge my 
tion of the newer eruptive rocks of Nantasket. His determina- 
tions have prevented serious errors in my work and enabled me 
to draw certain conclusions with a degree of confidence which it 
would otherwise have been impossible to feel. 
