J. H. Pratt — Occurrence of Chromite, etc. 281 



Akt. XXIX. — On the Occurrence, Origin and Chemical Com- 

 position of Chromite / by J. H. Pratt. 



Introduction. — In a recent paper* on the origin of the corun- 

 dum associated with the peridotites in Korth Carolina, 

 attention was called to the constant presence of the mineral 

 chromite in these rocks. During the past summer the occur- 

 rence of chromite has been carefully studied and the occur- 

 rences in North Carolina have been examined in the field. As 

 a result of these examinations the author has been led to adopt 

 the theory that the chromite should be regarded as having been 

 formed at the same time with the peridotite, i. e., as having 

 been held in solution by the molten mass of the peridotite and 

 crystallized out among the first minerals as the mass began to 

 cool. 



This theory is essentially the same as that advanced by 

 the authorf for the origin of the corundum associated with the 

 peridotite rocks, and a similar line of reasoning has been used 

 to substantiate the theory proposed. 



Investigations concerning the igneous origin of some of the 

 ores have been materially aided, during the recent years, by 

 the able experiments of Morozewicz^: and Lagoriog and by 

 researches that show us more clearly why we should regard a 

 fused mass of rock as a liquid, having similar properties to an 

 ordinary solution. The fused mass of rock is capable of hold- 

 ing different minerals in solution and as the molten mass begins 

 to cool, these minerals would separate out not according to 

 their fusibility but according to their solubility in the fused 

 mass. The more basic minerals being the more insoluble 

 would be the first to separate out, and as was mentioned in the 

 paper already referred to, this crystallizing or solidifying out 

 from the molten mass would take place first on its outer 

 boundaries, for here it would cool first. Convection currents 

 would tend to bring new supplies of material to the outer zone 

 where crystallization takes place. 



Occurrence. — With the exception of alluvial deposits, chro- 

 mite has only been found in the peridotite and allied igneous 

 basic magnesian rocks, or in the serpentines which have resulted 

 from the alternation of these rocks. In the North Carolina 

 peridotites chromite occurs more commonly as scattered grains 

 or crystals, but it is also to be found in the form of imbedded 

 masses near the boundaries of the lenticular bodies of dunite. 



*This Journal, vol. vi, p. 49, July, 1898. 

 fThis Journal, vol. vi, p. 50, July, 1898. 

 jzeitschr fiir Kryst , vol. xxiv, p. 281, 1895. 

 § Ibid., p. 285. 



Am. Jour. Sci.— Fourth Series, Vol. VII, No. 40.— April, 1899. 

 19 



