Relations of the Igneous Rocks — Crosby. Z7 
cient to cut through the conglomerate series and the inter- 
bedded andesite, toward the western end of the arch, where 
they were most elevated, and thus expose the ancient foun- 
dation of felsite and granite. 
Probably no phase of this study possesses a greater in- 
trinsic interest than the comparison of the denuded major 
axis of the Neponset anticline with the Blue Hills complex, 
which is but the denuded axis of the great anticline separ- 
ating the Boston and Norfolk basins; and aside from the 
disparity in area, it is surprising to find how marked is the 
similarity, except in minor details, and how few are the 
vital contrasts. In general terms, it may be stated that 
hardly anything is precisely similar in the two areas and 
nothing is radically different. In the smaller area as in the 
larger we have isolated masses of Cambrian strata involved 
in a complex of post-Cambrian granitic rocks, including the 
normal granite, the contact zone of fine granite and quartz 
porphyry, the effusive felsites and the intersecting dikes of 
diabase of several different systems. The chief contrast is 
found in the relatively greater abundance in the Neponset 
complex of the effusive felsites, their more varied character, 
the great profusion of dikes of felsite in the granites, the 
more positive identification of some of the principal vents 
or points of emission of these acid lavas, and the far more 
complete and clearer exhibition of their relations to the 
later basic lavas and the inclosing Carboniferous strata. 
Among the problems of special interest presented by 
the Neponset anticline and, apparently, admitting of suc- 
cessful determination, may be mentioned : the detailed rela- 
tions of the rocks of the basal complex ; the mutual rela- 
tions of the acid and basic lavas — rhyolite (felsite) and 
andesite ; and the relations of both types of volcanics to the 
THE BASAL COMPLEX. 
The basal complex may best be defined as comprising 
all of the pre-Carboniferous terranes of this region, both 
sedimentary and eruptive ; or more specifically, as consist- 
ing of the Cambrian strata and any other pre-Carboniferous 
and pregranitic sediments which future investigation may 
prove to exist here, together with the intersecting and 
associated igneous rocks of pre-Carboniferous age, includ- 
