38 The American Geologist J,,, - v - li,o:> 
ing the normal granite or main body of the batholite, and 
its contact zones of diorite, fine granite and quartz por- 
phyry, and the dikes, necks and Mows of acid lavas or fel- 
sites. As thus defined, the igneous part of the complex is 
clearly the product of the chemical and textural differenti- 
ation of a single great body of magma, embracing, besides 
the truly plutonic mass or batholite proper, developed, with 
its variable contact zone, under and in the Cambrian strata, 
the intrusive and effusive masses evolved, after extensive 
erosion of the Cambrian cover, from either still unsolidified 
or remelted deep-seated portions of the batholite. 
That the batholite, with the complicating sedimentary 
and igneous phases, which gives it the character of a true 
and typical complex, is continuous under all the newer 
formations of the region and, in its successive phases, essen- 
tially contemporaneous throughout, is highly probable; and 
the variations observed from point to point must, there- 
fore, be regarded either, as actual and due in part to differ- 
ences in the original magma resulting from the fusion of 
the pre-Cambrian Moor and in large part, also, to the vary- 
ing thickness and composition of the original Cambrian 
cover, or as merely apparent and due to the varying depths 
of pre-Carboniferous and post-Carboniferous erosion, or 
again, as due to the localization of the intrusive and effusive 
phenomena which followed the formation of the batholite 
proper, adding greatly to its structural complexity. 
Tf, with these ideas in mind, we compare more partic- 
ularly than heretofore the portion of the basal complex 
rising westward from beneath the Carboniferous sediments 
of the Neponset valley with the portion exposed, as the 
result of still more extensive erosion, in the Blue Hillsarea. 
we find the more notable differences to be as follows: First, 
the normal granite of the Neponset valley is prevailingly 
coarser grained and the ferromagnesian constituent (chiefly 
hornblende) is more generally and extensively altered 
(chloritized). Second, the differentiation of the contact 
zone appears to have been almost wholly textural, and not, 
to any important extent, chemical, in the Neponset Valley 
district ; and hence we find here only traces of diorite 
(which is also true of the Blue hills) and nothing closely 
