1894] Mineralogy and Petrography. 515 
intergrowths of albite and orthoclase cemented by granophyric quartz. 
It is unfortunate that the author cannot further pursue the studies so 
auspiciously begun. 
The Differentiation of Rock Magmas,—In a recent number 
of the Journal of Geology are two contributions relating to the 
theory of the differentiation of rock magmas. One, by Iddings,° 
is a simple statement of the nature of the phenomena that have 
led to the proposal of the theory. The article does not discuss 
the causes of the differentiation of magmas except in general terms, 
but it deals with the facts that seem to indicate that such a differentia- 
tion of a homogeneous magma into unlike parts is alone capable of 
accounting for the great differences observed in the various rocks 
emanating from a single volcanic center, and in different portions of 
the same rock mass, The second article, by Backtsrém‘, was written 
to call attention to the difficuly of explaining magmatic differentiation 
upon Soret’s principle, which applies, so far as we know, only to dilute 
solutions, and effects only the proportions existing between the solvent 
- and the dissolved body in different portions of a solution. The author 
prefers to consider rock magmas as mixtures of liquids, some of which 
_ are less soluble in others at certain temperatures than at certain dif- 
ferent temperatures. Hence if a homogeneous magma cools to a 
temperature when some of its constituents become difficultly soluble in 
the mixture of the others, it will become separated into pori amend 
ing different compositions—liquation will ensue. Thus basic concre- 
tions are sometimes formed in acid rocks, and the acid and the basic 
lavas of Iceland occur in numerous flows, side by side, while interme- 
diate rocks are absent. 
The Old Volcanics of South Mountain, Pennsylvania.— 
Miss Bascom? has examined with great thoroughness the acid volean- 
ics of South Mountain, Pa., whose existence was made known to the 
geological public a year® ago, and has described briefly the — of 
cher study. These volcanics exhibit many of the features of T SF 
rhyolites in spite of the fact that they have undergone n = 
alteration since their eruption. Fluidal, micropoieilitic, sp erulitic, 
axiolitic and lithophysal structures are noticed in the various speci- 
*Jour. Geol., Vol. I, p. 833. 
‘Ib., Vol. I, p. 773. 
‘Jour. Geol., Vol. I, p. 813. 
SAmer. Jour. Sci., XLIV, p. 482. 
