148 Tlie American Geologist. March, 1897 
their agreement with tl]e latter in composition and rehitions 
to the inclosing formations, has led many writers to regard 
the pegmatite itself as of plutonic igneous origin, it is not 
long since geologists, from a consideration chiefly of the ex- 
ceedingly coarse crystallization and frequent comb-structure 
of the pegmatites, were quite generally united, under the 
leadership of T. Sterry Hunt, in the conviction that they were 
true vein rocks, due to the deposition of the various compo- 
nent minerals from solution in open tissures or other preexist- 
ing cavities. Now. however, a decided drift in the opposite 
direction may again be noted, and the recent literature of the 
science indicates an approaching consensus of opinion in fa- 
vor of the association of the pegmatites, in their genetic re- 
lations, with plutonic igneous rocks rather than with subter- 
ranean aqueous deposits. In fact, the view that pegmatite is 
in some sense an intrusive rock may be regarded as so well 
established, through the labors of Lehmann,* Brogger,| Wil- 
liams, J and others, that the theories of lateral secretion and 
aqueous deposition are fast becoming of merely historical in- 
terest. 
This modern view is more acceptable than any of its prede- 
cessors mainly because it is less extreme. It is a reaction 
toward the igneous category, but the pendulum has not swung 
so far as to wholly exclude the aqueous influences. In short, 
the new theory is not simpl}' a revival of the old, but it is es- 
sentially a combination or compromise — neither igneous nor 
aqueous, but aqueo-igneous. The modern conception is that 
in a broad view the pegmatites are igneous rocks, but it is the 
]iart which water has played in their formation that has so 
strongly differentiated them from other igneous types. Al- 
though we shall incidentall}^ 'ipp'.y the touchstone of observed 
facts to all the principal hypotheses, the chief purpose of this 
paper is a contribution to the aqueo-igneous theory. § 
=^Ueber die Entstehung der alt krystallinischen Schiefergesteine.1884, 
p. 24 ei neq. 
tDie Mineralien der Syenitpegmatitgange der siidnorwegischen Augit 
und Nephelinsyenite, i, Theil. pp. 215-225. For translation, by N. N. 
Evans, see Canadian Record of Science, 6, 33-46 and 61-71 
J"Origin of the Maryland Pegmatites." Fifteenth Annual Report 
United States Geological Survey, pp. 675-684. 
§The contributions of Brogger and Williams, already referred to, are 
replete with references and render citations of the earlier literature un- 
necessary here. 
