282 J. H. Pratt — Occurrence, Origin and 



It does not occur in well defined veins, but is often in masses 

 or pockets which apparently have no relation whatever to 

 each other. But few writers on the occurrence of chromite 

 have described the relation of the chromite deposits to the 

 rock in which it is found. One or two have mentioned the 

 chromite as being found near the eastern boundary of the ser- 

 pentine or at the northern border of the serpentine belt, but 

 no definite description of the occurrence could be obtained. 

 The large deposits of chromite in ISTorth Carolina occur in the 

 peridotite rock and near the contact of this rock with the 

 enclosing gneiss. Also where there is but a small amount of 

 chromite either in small pockets or in grains or crystals, -these 

 are more abundant near the contact and diminish in number 

 toward the center of the mass of the peridotite. 



Where these large deposits of chromite occur there has been 

 no corundum or but very little found, and where we find the 

 large deposits of the corundum there is a scarcity of chromite. 



This constant occurrence of the chromite in rounded masses 

 of varying proportions near the contact of the peridotite with 

 the gneiss, its occurrence in the fresh as well as in the altered 

 peridotite, indicate that the chromite has been held in solution 

 in the molten mass of the peridotite when it was intruded 

 into the country rock and that it separated out among the first 

 minerals as the mass began to cool. 



As has been said, the peridotite (dunite) magma holding in 

 solution the chemical elements of the different minerals would 

 be like a saturated liquid, and as it began to cool the minerals 

 would separate or crystallize out, not according to their fusibil- 

 ity but according to the degree of their solubility in the molten 

 magma. The more basic minerals, according to the general law 

 of cooling and crystallizing magmas, being the less soluble, 

 would therefore be the first to separate out. These would be 

 the oxides containing no silica and in the present case would 

 be the chromite, spinel and corundum. These minerals would 

 solidify or crystallize out where the molten magma first began 

 to cool, which would be at the contact of the mass with the 

 country rock ; convection currents would tend to bring new 

 supplies of material to the outer boundary which would deposit 

 its chromic oxide as chromites. This would account for all the 

 irregularities of the chromite deposits : their pocket-like nature, 

 the shooting off of apophyses from the main masses of the 

 chromite into the peridotite, the widening and pinching of the 

 chromite lodes, and the apparently non-relation or connection 

 of one pocket of chromite with another. There has not been 

 enough work done in the North Carolina chromite mines to 

 demonstrate the exact position and relation of the chromite 

 deposits to the gneiss or the country rock, and in the descrip- 



