PETROLOGICAL ABSTRACTS AND REVIEWS 715 



and olivine. For example, in sulphide-rich melts with melilite, much of the 

 mass of sulphide crystallizes out before the corresponding spinel, and the 

 remainder crystallizes synchronously with the spinel. 



In a solution of calcium magnesium silicate, calcium sulphide in the amount 

 of 2.3 per cent lowers the melting point about 50°. By calculating the molecular 

 depression it is found that polymerization of the sulphides of calcium and 

 manganese in silicate melts is essentially nil; in fact, in the silicate solutions 

 an extensive electrolytic dissociation more probably takes place. 



Somewhat similar results as to the presence of a eutectic in solutions of 

 zinc, aluminum, and iron oxides are discussed. In a melt bearing zinc sulphide, 

 zinc spinel, and meliUte, the order of crystallization fellows the order of nam- 

 ing, as above; if, however, olivine be present instead of melilite, and only a 

 very little sulphide, the order of crystallization is spinel, olivine, and sulphide. 



In other slags the presence of copper and its relation to chondri-like 

 structures, and to iron sulphide-bearing sihcate solutions were studied. An 

 interesting feature is the concentric arrangement of iron sulphide inclusions 

 in hexagonal plates of biotite. From these observations it is found that CuaS 

 is wholly insoluble or at best only slightly soluble in sihcate melts rich in iron 

 sulphide — attributable possibly to the presence of a common ion. Various 

 conclusions drawn in this part of the investigation are interesting not only to 

 the geochemist, but to the metallurgist as well. 



Some space is also devoted to the crystallization of apatite and ore — the 

 so-called " telechemical " minerals — those only distantly related to the silicate 

 minerals. 



The foregoing tacts may be gleaned from the publication of 1919. The 

 paper of lyiy briefs most of these observations, and adds considerable material 

 on the part of sulphides in eruptive magmas — an appUcation of physical 

 chemistry to systematic petrogenesis. 



Pyrite appears to crystallize very early from magmas, and has a higher 

 melting point than magnetite. Pyrite generally precedes pyrrhotite in the 

 normal sequence of crystallization; it also precedes chalcopyrite, which gener- 

 ally follows magnetite when all three sulphides crystallize from a melt. As 

 previously observed, magnetite is only very slightly soluble in pyrite-rich 

 melts; hence iron oxides are rare or absent in such melts. 



A study of the norite-type magmas leads to the conclusion that the pre- 

 dominant mineral generally crystallizes first; thus, in plagioclase-rich norite, 

 plagioclase is automorphic; in olivine-plagioclase, plagioclase-rich rock, plagio- 

 clase; in olivine-rich olivine-plagioclase rocks, olivine is automorphic, and so 

 on. The physical chemistry of the several systems normally present in a gab- 

 bro-norite magma is summarized, and the writer believes in a eutectic for 

 rocks of the gabbro-norite group, this eutectic, however, being expressed by a 

 line, rather than by a point. 



The rather constant appearance of pyroxenites and peridotites in close 

 association with nickel-pyrrhotitic deposits, as at Sudbury, is thought to be 



