THE NONHETALLIC MINERALS. 221 



cleavage (Specimen No. 63480, U.S.N.M., from Pine Mountain, 

 Georgia). 



The most striking physical property of the mineral is its hardness, 

 which is 9 of Dana's scale. In this respect it ranks then next to the 

 diamond. The color of the mineral varies from white through gray 

 (Specimen No. 46283, U.S.N.M), brown, yellow, blue (Specimens Nos. 

 73531 and 48182, U.S.N.M.), pink (Specimen No. 81922, U.S.N.M.), 

 and red; luster adamantine to vitreous; specific gravity, 3.95 to 4.1. 

 The highly colored transparent red and blue forms are valuable as 

 gems, and are known under the names of ruby and sapphire. The 

 consideration of these forms is beyond the limits of this work. (See 

 Mineral and Gem Collections.) 



Occui'rences. — Athough widespread as a mineral, corundum, unmixed 

 with a large proportion of magnetite (forming emery), has been found 

 in but few localities in sufficient abundance to be of commercial value. 

 The most important deposits in the United States are in southwestern 

 North Carolina and in the Laurel Creek region of northern Georgia. 

 The country rock in both these regions is hornblendic gneiss, through 

 which has been intruded a basic eruptive (dunite, Specimen No. 70069, 

 U.S.N.M.), and it is mainly along the decomposed lines of contact 

 between the two that the corundum is found. According to Dr. T. M. 

 Chatard, the Corundum Hill Mine is situated on a ridge which runs in 

 the northeast and southwest direction characteristic of this section, the 

 dunite outcrops being on the crest, and apparently surrounded on all 

 sides except toward the east by hornl)lende gneiss. On the east side 

 mica schist (probably damourite schist) takes the place of the gneiss, 

 and it is on the eastern side of the dunite that the so-called '' sand vein" 

 is found. This is a vein-like mass of brown vermiculite in small scales 

 containing an abundance of small crystals of corundum which are usually 

 brown in color and often broken into fragments (Specimen No. 73529, 

 U.S.N.M.). The easterly wall of this vein is the mica schist very 

 much decomposed, while on the western side is found enstatite (Speci- 

 men No. 70070, U.S.N.M.), next vermiculite mixed with chlorite, then 

 talc (Specimen No. 70071, U.S.N.M.), which in turn gives place to 

 nodules of more or less altered dunite. 



The specimens of corundum crystals for which this locality is so 

 celebrated (Specimen No. 73530, U.S.N.M.) have been found mainly, if 

 not wholly, on the westerly side of the dunite, and on or near the line 

 of contact between the gneiss and dunite. 



State Geologist Yeates has stated^ that in the Laurel Creek region 

 the corundum is not confined to the vermiculite and chlorite bands, 

 but is abundant in the lime soda feldspar as well. The same authority 

 states that in this region the dunite is not inclosed by the hornblendic 



1 Bulletin No. 2, Geological Survey of Georgia, 1894. 



