152 The Americcni Geologist. March, i»97 
Williams lay much stress upon this fact and cite abundant 
illustrations of it, noting especially that the pegmatite is more 
acid than the parent mass. Williams says of the pegmatite 
dikes of Maryland, ''They agree essentially in chemical and 
mineral composition with the granite masses whose igneous 
origin is well established, although they are in the main some- 
what more acid than these, and their size and abundance are 
directly proportioned to their nearness to some eruptive gran- 
ite mass. At many lo(?aIities they can be seen to decrease 
steadily both in number and size, as they recede from such a 
granitic boundary." The more acid character of pegmatite 
than of granite is seen in the fact that among the micas bio- 
tite largely predominates in granite, and is the exception in 
pegmatite. 
The parent plutonics of the pegmatites which we have stud- 
ied in New Hampshire are the Concord granite, and the Mon- 
talban gneiss, the latter seeming to be merely a more gneissoid 
phase of the former. These two granitic types, it may be 
added, are readily proved b^' the field evidence to be the 
youngest and most acidic of the entire granite series of the 
region, the fact pointing very plainly to a progressive chemical 
differentiation of a vast body of magma during a long period, 
with, at the end, a uiarked textural differentiation. In New 
Hampshire we have not found the pegmatite so abundant in 
the Concord granite or an}"- plutonic formation as in the tibro- 
lite and other schists, which are easily proved to antedate all 
these igneous rocks. This is due in part, probably, to the 
fissile and flexible character of the schists, by virtue of which 
they offer an easier passage to intrusives than do the more 
massive formations; and in part, no doubt, to the relative 
readiness with which they are dissolved or melted away by 
the invading magma. It may be added that in the schists, as 
in the gneisses and granites, the amount of pegmatite is, in 
general, inversel}'' proportional to the distance from the Con- 
cord granite. 
It is a reasonable expectation that the absorption of great 
bodies of schist should atfect the composition of the pegma- 
tite. In fact, Geikie says [Text- Book of Geology, p. 302) 
igneous rocks dissolve or eat away the inclosing formations, 
and in consequence vary considerably in composition from 
