1894.] Petrography. 949 
Petrographical News.—In a series of articles recently published 
Vogt? diseusses the formation of oxides and sulphide ores around basic 
eruptive rock bodies, describes all the known occurrences of the nickel 
sulphides with reference to their mode of origin, and reviews critically 
the literature treating of the differentiation of rock magmas. e 
shows that the nickel ore deposits that are peripheral must be due to 
differentiation of rock magmas. He further shows that the laws gov- 
erning the processes of differentiation are very complieated and that 
neither Soret's principle nor any other single physical or chemical 
principle will satisfactorily explain the phenomena. 
Dr. G. H. Williams? reports the occurrence of volcanic rocks at 
many localities in the eastern crystalline belt of North America. The 
rocks in question comprise tuffs, glass breccias, devitrified obsidians and 
fine grained crystalline flow rocks with many of the characteristics of 
modern lavas. All these have heretofore been regarded as sedimentary 
in origin by most of the geologists who have studied them. The 
author gives his reasons for concluding that they are volcanic, and de- 
clares that, not before their true character is recognized will the struct- 
ure of the crystalline areas of the Appalachians be correctly under- 
stood. 
Lang" discusses the conclusions of Rosenbuch" with respect to the 
chemical nature of the crystalline schists, and criticizes Linck’s prin- 
ciples governing the mineralogical composition of eruptive rocks. In 
his article, which is well worth reading, the author shows conclusively 
that the mineral composition of rocks is -not determined by their 
chemical composition. 
*Zeits f. prakt. Geol., 1893, Jan., April. 
?Journ. of Geol., Vol. 2, p. 1. 
‘Min. u. Petrog. Mitth., XIII, p. 496. 
UCf. American Naturalist, 1891, p. 827. 
