Review of Rece?it Geological Literature. 327 
tribution or geological occurrence. Even the high sulphur oils, which 
some have regarded as peculiar to limestones, are shown to be also 
very typically developed in sandstones and shales. The importance and 
feasibility of a general system of classification on the basis of the series 
of hydrocarbons which constitutes the main body of the crude oils arc 
discussed at some length. w. o. C. 
The Genesis of Bitumetis, as Related to Chemical Geology. By S. 
F. Peckham. Proc. Am. Phil. Soc, 37, 108-139. 
This paper is a sequel to the preceding symposium; but the author 
contents himself with a very general discussion of the geology of the 
bitumens and especially of petroleum, and a restatement of the facts 
supporting the distillation theory of petroleum and natural gas. 
w. o. c. 
On Dikes of Felsophyre and Basalt in Paleozoic Rocks in Central 
Appalachian Virginia. By N. H. Darton. Notes on the Petrography 
By Arthur Keith. Am. J. Sci., 156, 305-315. 
A third locality of igneous rocks in the Central Appalachian region 
has recently been discovered by Mr. Darton. Both basic and acidic 
dikes are represented, the latter belonging to the granite family and 
most nearly resembling felsophyre. Petrographic descriptions of both 
types are given by Mr. Keith, supplemented by an analysis of the felso- 
phyre by W. F. Hillebrand. M. l. f. 
Platinum and Iridium in Meteoric Iron. By John M. Davlson, 
Am. J. Sci. 157, 4. 
After treating the Coahuila and Toluca meteoric irons with hydro- 
chloric acid, there remained in each case a fine black residue consisting 
of irregiilar or tetragonal crystals of rhabdite, carbon, and some stony 
matter. Analyses of these sediments showed the presence of both plat- 
inum and iridium. From 608.6 grams of Coahuila iron there were ob- 
tained 0.014 gram of platinum, and 0.0015 gram of black powder, prob- 
ably iridium. The amounts obtained from the Toluca iron were con- 
siderably less. Unsuccessful search was made for microscopic dia- 
monds. 'M. L. F. 
Causes of Variation in the Composition of Igneous Rocks. By T. 
L. Walker. Am. J. Sci., 156, 410-415. 
A brief review is given of the usual theories of magniatic differentia- 
tion. The author considers that none give satisfactory explanations of 
all the phenomena observed, especially of the excessively basic centers 
of certain eruptive masses. Attention is called to the fact that certain 
homogeneous solutions remaining for a long time at constant temper- 
atures become gradually more concentrated in the lower portion, and 
it is urged by the author that a similar concentration probably occurs 
in complex silicate magmas, especially when near the temperature of 
consolidation. The material along the borders solidifies before the 
differentiation is far advanced, representing the original composition. 
Differentiation and solidification progress simultaneously, and hence 
the central portions will have suffered the greatest changes. In the 
