216 The American Geologist. Maroh,i8&n 
body of granite magma, at a great depth in the earth, the water in the 
magma is gradually forced inward, the magma residuum becoming thus 
more highly hydrated and more liquid. Fusion passes without a break 
into solution; and the increasing mobility of the unsoliditied magma 
finally permits the development of the coarse and perfect crystalliza- 
tion, comb-structure, vugs or pockets, and other characteristic features 
of typical pegmatite. Pegmatite may thus be defined as a highly dif- 
ferentiated portion of a granite magma, the water, which is an essential 
feature of the magma, becoming concentrated in a limited portion dur- 
ing solidification. The amount of water being quite insufficient to hold 
the minerals of the magma in simple solution, they crystallize out long 
before the magma reaches that condition. 
Soret's principle, upon which Iddings bases his argument for the 
general differentiation of igneous rocks, that if the temperature of a 
solution is not uniform the dissolved substances will become concen- 
trated in the cooler part of the solution, also finds application here. 
The cooler part of a body of magma is the periphery, and toward this 
the dissolved minerals are concentrated, the central and hotter portion 
of the magma or solution thus becoming steadily thinner — poorer in 
dissolved minerals and richer in the solvent (water)— until at last the 
conditions favor the coarse and perfect crystallization of the pegmatites. 
Spherulitesandlithophysa? are believed to present analogous phenomena 
on a minute scale; and an instructive analogy may also be traced be- 
tween the development of pegmatite masses with pockets in the center 
of a granite boss and the formation of an ordinary quartz geode, with 
its characteristic chalcedonic shell, crystalline interior, and central 
pocket. The pegmatites are intermediate, in origin, between plutonic 
or dike rocks and vein rocks, combining the characters of both; and 
the author holds that the true veins are entitled to recognition in the 
lithological classification. 
46. A classification of economic geological products, based uponori- 
gin and original structure. Wllliam O. Crosby, Boston, Mass. This 
paper will be presented in full in an early number of the American 
Geologist. 
/;. Lake Cayuga a rock basin. R. S. Tarr, Ithaca, N.Y. (Read by 
title.) After reviewing the literature relating to the origin of the Fin- 
ger lakes of central New York, evidence is presented to show that lake 
Cayuga, having a maximum depth of 43o feet, is in a rock basin. This 
evidence is obtained from a study of the preglacial tributaries, which 
at present empty into the lake in completely enclosed rock valleys 
whose rock bottoms are above the present lake surface. Lake Cayuga 
is therefore in a preglacial valley, which has been deepened during the 
Glacial period at least 450 feet at one point. 
Evidence is also presented to sbow that the preglacial Cayuga river 
flowed northward, being tributary to the preglacial Ontario stream. It 
seems accordingly an inevitable conclusion that lake Ontario is also a 
rock basin. The conclusion is thus reached that no evidence is afforded 
here in favor of continental elevation and depression ; and it is also 
pointed out that, this being the case, an outlet for the preglacial great 
lake basin drainage through the Mohawk and Hudson is more probable 
than it has hitherto appeared. 
t8. Pleistocene problems in Missouri. James E. Todd, Vermillion, 
S. D. (Read by Mr. L. S. Griswold.) This paper presented some of the 
data recently collected by the author in exploration for the Missouri 
Geological Survey, and attempted to harmonize them with similar facts 
from adjacent regions. The data comprise descriptions of the pregla- 
cial deposits: of the character and distribution of the bouldery drift, 
or till, and of the loess and its associated deposits; of the terraces, 
ancient channels and rapids of the Missouri and other streams; and off 
the present topography as related to the Pleistocene formations. The 
