Origin of Pegmatite. — Crostyy and Fuller. 1G5 
heated to a high temperature and that, therefore, the cooling 
must have taken place unusually slowly and uniformly; and 
upon this fact primarily the largeness of the grains may be 
explained." 
This explanation of these extremely important characters, 
which no theory of pegmatite can ignore, appears to us singu- 
larly inadequate, in view of the following considerations: 
(1) The surrounding rocks must always be cooler than the 
great body of granitic magma by which they are heated. (2) 
The coarsest pegmatite is often in narrow dikes thousands of 
feet if not miles distant from the border of the plutonic mass 
in which it originated. (3) If, as Brogger insists and we be- 
lieve, p'3gmatite is the end-product or final crystallization of 
the original granite magma, we are, to paraphrase his criti- 
cism of Lehmann, (p. 43) led to the remarkable result that a 
body of magma solidifying as 'the result of cooling has a 
higher temperature and therefore crystallizes more slowly and 
coarsely in the later than in the earlier stages of the process. 
(4) Brogger holds that the most typical pegmatite may be, 
and in special instances cited by him must have been, formed 
at very moderate depths below the earth's surface — a few 
hundred feet to two or three thousand feet. How, under these 
conditions, the vast body of granite magma at greater depths 
and cooling far more slowly escaped conversion into pegmatite 
is a mystery which he nowhere offers to explain. 
But he goes on to say* "That this explanation of the coarse 
grain and of the imperfect zonal structure of many peg- 
matite veins is correct, is rendered probable in the highest 
degree by the frequent occurrence of pegmatite structure in 
those portions of rock bordering on the open drusy cavities 
of many massive granites. I interpret these as analogous to 
the formation of the pegmatite veins themselves, in the fol- 
lowing way: First, on account of the contraction due to 
crystallization of the rock already for the most part solid- 
ified, there were formed crystal-free luniina (microlitic struc- 
ture on a large scale) : the mixture of magma and crystals so 
formed, which must have constituted a somewhat solid rock, 
inclosing the lumina, was, however, completely permeated by 
the magma, and with this these crystal-free spaces would nat- 
*L. c. G7. ~~~ 
