40 The American Geologist. 
July, 1905 
heat invasion was, doubtless, accompanied by a strong ele- 
vation of the surface, permitting extensive erosion, which, 
in turn, favored the refrigeration of the batholite and the 
development from the originally homogeneous magma of a 
vast body of normal granite, with a contact zone consisting, 
normally, of an inner layer of fine granite and an outer 
layer of quartz porphyry, both phases of the contact zone 
being the products mainly of a textural rather than a chem- 
ical differentiation of the magma. 
Long continued erosion, removing in large part the 
sedimentary cover of the batholite and probably cutting at 
some points through its contact zone into the normal gran- 
ite, was followed by a period of volcanic activity, due pos- 
sibly to cracking and hydration of the body of the batholite, 
during which, acid lava, chiefly rhyolite, now existing in 
a. devitrified form as aporhyolite or felsite, was poured out 
over the eroded surface of the batholite. Several of the 
volcanic necks or vents of these effusive eruptions have 
been definitely located and their details of form and struc- 
ture more or less fully worked out. From the vents or 
chimneys of these most ancient volcanoes of the Boston 
basin radial dikes of felsite extend outward into the granitic 
rocks. As a chronologically distinct record, the complex 
was now complete ; but it was destined to be still further 
complicated ; for these effusive acid eruptions appear to 
have marked the beginning of the progressive subsidence 
which inaugurated the deposition of the Carboniferous 
sediments, beginning with the great conglomerate series; 
and during the subsidence and clastic sedimentation the 
effusive eruptions continued, but became of more basic 
character — grading from rhyolite through trachyte to an- 
desite, which in its present altered form as apoandesite or 
porphyrite has been heretofore classed as melaphyre, but 
is now known to be less basic than that type. The ande- 
sitic eruptions, from, presumably, greater depths than the 
source of the acid lavas, are marked by fissurelike necks, 
by numerous dikes cutting all the older rocks, and espec- 
ially by successive massive flows or contemporaneous beds 
intercalated in the conglomerate series. 
The volcanic activity finally ceased and continued sub- 
