222 REPOET OF NATIONAL MUSEUM, 1899. 



gneisses, but intruded between these and other gneiss or mica schist; 

 also that the corundum-bearing veins lie in the dunite close to the con- 

 tact and in the vicinity of the hornblendic gneiss. It should be said 

 before leaving the subject that certain micaceous minerals, as margarite 

 and chloritoid (Specimen No. 63107, U.S.N.M., from Chester, Massa- 

 chusetts) are almost invariable accompaniments of corundum and 

 emery deposits, and that it was the finding of these minerals that led 

 to the discovery of the emery beds at Chester. Chatard reports that 

 in the North Carolina mines chlorite or vermiculite is considered a 

 "corundum sign," and in mining such indications are followed so long 

 as they hold out (Specimen No. 63153, U.S.N.M.). 



The geographical distribution of corundum-bearing rocks in the 

 eastern United States has been worked out in detail by J. V. Lewis of 

 the North Carolina Geological Survey, from whose report^ the accom. 

 panying map (Plate 8) is taken. According to this authority the 

 corundum occurring in such quantities as to be of commercial value is 

 almost universally found in connection with basic eruptive rocks, as 

 peridotites or their varietal forms pyroxenite and amphibolite, which 

 are themselves intruded into gneisses. 



At Yogo Gulch, Montana, corundum in the form of sapphire (see Gem 

 Collections) occurs as a constituent of a basic eruptive rock near the 

 line of contact with aluminous shales (Specimen No. 53519, U.S.N.M.). 

 In Gallatin County the mineral is found in well-defined crystals of all 

 sizes up to an inch or more in length abundantly disseminated through- 

 out a granite (Specimen No. 83838,U.S.N.M.). In the Russian Urals it 

 occurs in disseminated crystals and large cleavage masses in feldspar 

 (Specimens Nos. 40323, 40315, 40334, 73532, U.S.N.M.). In India it 

 occurs as an original constituent associated with both acid and basic 

 rocks, but in most cases where the mineral is in the basic rocks there 

 have been found intrusions of pegmatite (an acid rock) in the near 

 vicinity. In the celebrated Mogok Ruby Mines the corundum is found 

 in a crystalline limestone and the detritus resulting from its decay, the 

 limestone itself being regarded by Professor Judd as an extreme form 

 of alteration of rocks of igneous origin (see further under Emery). 



Corundum has recently been reported as a constituent of both nephe- 

 line syenites and ordinary syenites in the counties of Renfrew, Hast- 

 ings, and Peterborough, in Eastern Ontario, Canada. According to 

 W. G. Miller^ these syenites are dike rocks, consisting essentially of 

 feldspar, nepheline, and black mica or hornblende, the corundum 

 occurring more abundantly in the ordinary syenite than in that which 

 carries nepheline. The dikes are from a few inches to some feet in 

 diameter, and the corundum is distributed in a somewhat capricious 



^Bulletin No. 11. Corundum and the Basic Magnesian Eocks of Western North 

 Carolma, by J. V. Lewis, 1896. 

 =^ Report of the Canadian Bureau of Mines, VII, Pt. 3, 1898, p. 207. 



