1921] NATURAL SCIENCES OF PHILADELPHIA 191 



work indicate that corundum would not be expected as an early 

 phase in the crystallization of a dry magma near peridotite in com- 

 position. It is clear that evidence of magmatic segregations of cor- 

 undum in ultrabasic rocks is lacking. 



On the other hand the presence of abundant tourmaline, much 

 plagioclase, with garnet, zoisite, vermicuhte, etc., is indicative of 

 pegmatitic assemblages, and it is therefore more prol)able that 

 these corundum veins represent granitic pegmatites which have so 

 reacted with the wall rocks as to almost lose their identity. The 

 contact zones are somewhat different from those of the plumasites 

 described above, but wide variations are to be expected depending 

 on the composition of the original pegmatitic solutions, and even 

 more so on the character of the peridotite or pyroxenite, and on 

 whether these had already been hydrated to serpentine. It may 

 be noted that much magnesia was assimilated by the pegmatitic 

 solutions with the formation of much chlorite and biotite (since al- 

 tered to vermiculite) and some spinel in the corundum veins. The 

 distinct chloritic zones are evidence of reaction with the wall rock, 

 whereby alumina was furnished by the pegmatitic solutions, and 

 magnesia' by the peridotite. The zone of enstatite may be ex- 

 plained by silication of the olivine, perhaps by the reaction 

 Mg.Si04"rSiO.=2MgSi03 or the mineral may have been produced 

 by abstraction of some of the magnesia. Incidently it is rather unus- 

 ual to find enstatite rather than anthophyllite. The position of the 

 talc zone between the dunite and the enstatite is anomalous, but it 

 might be noted that most of the analyses of the North Carolina dun- 

 ites show the presence of water, and more or less alteration to serp- 

 entine. 



Summary of Mineralogy of Desilicated Granitic 



Pegmatites. 



The mineralogical composition of this group depends largely on 

 the composition of the original pegmatitic solutions, that of the 

 wall rock (peridotite, pyroxenite, serpentine, etc.), and the degree 

 of desilication or extent of reaction between the two. Variations in 

 the character of the zones also occur. In some cases different per- 

 iods of mineralization, some distinctly later than others, may occur. 

 The minerals, exclusive of those occurring in the contact zone, 

 which are more properly to be regarded as contact metamorphs of 

 the peridotites, serpentines, etc., are listed below. 



