Origin of Pegmatite. — Crosby and Fuller. 
161 
Fig. 7— Vertical Section of Pegmatite and Overlying Schist, Euggles Mine, 
Grafton, N. H. 
tion*. Still a third cause must be invoked where the pegma- 
tite is inclosed in, and blends with, the parent plutonic rock. 
Although, as indicated, certain more or less definite terms 
may be applied to the pegmatite masses, the great majority 
are decidedly irregular and indefinite in form. In every in- 
stance the pegmatite is clearly younger than the foliation of 
the inclosing rocks. The masses vary in size from almost 
microscopic veinlets, which, as Williamsf has pointed out, 
show a high degree of liquidit}'^ in the original magma, up to 
bosses or dikes 500 feet or more, in width. So far as we have 
observed, the largest masses show a tendency to the rectangu- 
lar type. These include Mt. Tug, Orange, 500 X 1,500 feet; 
Osgood's Ledge, Acworth, 175 X 700 feet; Palermo mine, 
Groton, 200 X 500 feet. 
vSome of the rectangular forms are probabl}" blunt lenses. 
The lenticutar form is also very typically developed in some 
of the larger masses, such as the great veins on Ci\ystal moun- 
tain, Lempster, 80 X 800 feet, and Trow's hill, Sunapee, 50 X 
400 feet. We have already noted that the pegmatite dikes 
become smaller and fewer away from the parent mass of gran- 
ite. The minor lenticular veins, isolated lenses or eyes, 
conforming with the foliation of the schists, present an inter- 
esting problem as to the source and mode of introduction of 
the component minerals (Fig. 8, PI. ix). 
♦According to the observations of Delesse, volcanic rocks, when re- 
duced to a molten condition, attack briskly the sides of the Hessian 
crucibles in which they are contained and even eat through them. 
tU. S. Geo!. Survey, Annual Report, 15, G81. 
