The ^^ A i([/(')i^' -Gneiss Area. — Luqner and Uies. 261 
'• (Teneral Relations of the Granitic Rocks in the Middle At- 
lantic Piedmont Plateau.''* 
In this pa]ier Prof. Williams mentions the theories advanced 
l\y different writers and comments on the probable correctness 
of the acjueo-igiieous origin of pegmatites, as advocated by 
Lehmann, Brf'jgger and others. Regarding the source of the 
material, the lateral secretion process seems to be the one ac- 
cepted by most writers for the origin of the material. 
In Maryland, where the pegmatites are esi)ecially abundant. 
Prof. Williams considers that there are examples of both seg- 
regatif)nal and intrusive ones. The former are generally lens- 
shaped like many (juartz veins, which have evidently formed 
in the same manner, and they also show a banding parallel to 
their walls. In the intrusive pegmatites there is no banding, 
the boundaries are sharp and the character of the dike does 
not seem ti> bear any relation to the chemical composition of 
the wall rock. Their structure is usually coarse, but tine- 
grained phases may occur. Their igneous origin is shown b}'' 
the manner in which they branch and cut across the strike? 
and include fragments of the wall rock. The liquidity of the 
rock, as well as the pressure under which it was erupted, is 
inferred from the minute fissures which it sometimes fills. 
Intrusive pegmatite dikes may be concordant with the folia- 
tion of the gneiss, on account of their having followed the 
line of least resistance. 
In the case of the Bedford pegmatites it seems hard to con- 
ceive that such coarsely granular masses of quartz, mica and 
feldspar could be formed from igneous fusion, or that large 
and comparativel}^ pure masses of (juartz, many feet in width, 
should be the result of igneous ])rocesses. A hydro-thermal 
origin seems more likely, and the occurrence of abundant 
tourmaline also indicates the presence of mineralizers. 
A similar theory regarding the origin of the larger veins of 
pegmatite in metamorphic regions has been suggested by J. 
F. Kemp,f who says : "It seems improbable that true igneous 
fusion could have afforded such coarsely crystalline aggre- 
gates and so we are forced to assume such an abundance of 
steam and other vapors as to almost if not quite imply solu- 
tion." 
*ir)th Ann. Rept. Director U. S. Geol. Surv., p. 657. 
tLecture Notes on Rocks, S. of Mines Quart., xvii, No. 2, 1896. 
