Origin of Peyrnaiite. — Crosbi/ and Fidler. 149 
The term pegmatite has been employed heretofore in two 
senses: To clesignite (1) the macro- and micro-pegraatitic or 
graphic-granite structure in rocks; (2) a distinct type of 
coarsely crystalline plutonic rocks, which is characterized 
only to a slight degree by the pegmatitic or graphic structure, 
but is especially distinguished, as a rule, by the gigantic scale 
of its crj^stallization. It is, of course, in the second sense 
that the term is employed here. We desire, however, to direct 
attention at the outset to two prominent facts which appear 
to us to demand a modification of the definition. First, the 
pegmatites have a wide range in composition, from the most 
highly acidic of all rocks (pure quartz) to ultra-basic varie- 
ties, giant granite being an appropriate trivial name only for 
some of the more acid forms. Second, the dilferences between 
the pegmatites and the more normal plutonic rocks are tex- 
tural to a much greater degree than mineralogical or chemical. 
In other words, since the classification is based primarily 
upon composition (the result of magmatic differentiation), 
pegmatite should be regarded, not as a distinct species or 
family of rocks, but rather as a possible textural phase of all, 
or nearly all. the plutonic rocks; and we ma}' in general say 
of any plutonic rock that it may be crA-ptocrystalline, micro- 
crystalline, macrocrystalline, or pegmatitic. Our problem is, 
then, to account for the pegmatitic phase of the plutonic 
rocks. While there can be little or no doubt that the acid 
and basic pegmatites have had a common origin, every stu- 
dent of geology knows that the acid types have been far more 
generally and extensively observed and exploited, and hence 
more fully and satisfactorily described, and our own field 
studies, as already indicated, have been limited chiefly to this 
class. The conclusions stated in the following pages are to 
be understood, therefore, as applying especially to the acid 
pegmatites. 
Characters of the Acid Pegmatites. 
A complete description of the acid pegmatites is not in- 
tended here, but simply an enumeration of the chief features 
which liny satisfactory theory of their origin must explain. 
Coviposttion. — The acid pegmatites, like the corresponding- 
normal plutonic rocks, consist of anhydrous silicate species, 
and usually more or less of free acid (quartz) and traces of free 
