172 The American Geologist. March. 1897 
valuable essa}'' on the origin of igneous rocks*, as a cause of 
the differentiation of purely igneous magmas. Soret demonstra- 
ted that if two parts of a solution of any salt be kept at dif- 
ferent temperatures there will be a concentration of the salt 
in the cooler part of the solution. Iddings holds that in any 
molten magma certain oxides play the role of solvents for the 
others. But if Soret's principle admits of application to an 
anhydrous magma, much more should it be applicable to a 
magma in a state of aqueo-igneous fusion. In the granite 
boss the cooler part is the exterior, and toward the exterior, 
therefore, the dissolved bases will be concentrated, leaving in 
the interior a more highly hydrated portion, which will be in 
a state of igneo-aqueous fusion, and from which may be dev- 
eloped the coarse crystallization, comb-struc^ture, and pockets 
of t3q:)ical pegmatite. This principle probably co()perates 
with and is, perhaps, intrinsicall}^ more important than that 
of the exclusion of water by crystallization. The main point 
of this paper is the explanation of the p:\gmatites through a 
differentiation of acid magmas in which water plays an im- 
portant part, the differentiation involving for the magma re- 
siduum a transition from aqueo-igneous fusion to igneo-aque- 
ous solution. The possible intervention of extraneous water 
has been suggested, but this is not deemed essential. 
The increase in the degree of hydration of a small part of 
the magma, through the exclusion of water during solidifica- 
tion b}^ the remainder, is substantially the explanation wliich 
Iddings offers for the phenomena presented by spheratites and 
lithophysje. A moderate increase in the proportion of water 
about centers, the proximity of which will be deterjnined by 
the viscosity of the magma and the rate of cooling and solid- 
ification, gives the unsolidified residuum sufficient liquidity 
to permit the incipient r:idial crystallization of the sphervdite; 
while a higher degree of local hydration gives rise to the con- 
centric crystalline shells and shrinkage cracks (pockets) with 
the highly perfect crystallizations of feldspar, quartz, faya- 
lite, etc., characteristic of the typical lithophysae. 
An analogy may also be traced between this illustration of 
pegmatite and what appears to us to be the best explanation 
of the quartz geodes in certain limestone formations. Solu- 
*Bulletin of the Philosophical Society of Washington, 12, 158. 
