THE NONMETALLIC MINERALS, 223 



manner, being quite uniformly distributed in some of the smaller 

 dikes, or segregated irregularly along certain lines or patches. In some 

 of the dikes the mineral is quite lacking. The total area covered by 

 the corundum-bearing rocks, in the three counties mentioned, is 100 

 square miles (Specimen No. 53588, U.S.N.M.). 



Origin. — Dr. Chatard, as a result of his observations already quoted, 

 regards the corundum of Franklin County, North Carolina, and the 

 Laurel Creek region of Georgia as a secondary mineral produced by a 

 mutual reaction between the various elements of the dunite and 

 gneiss during decomposition, the solutions formed during such decom- 

 position giving rise to such reactions as are productive of chlorite and 

 vermiculites, and, where the necessary conditions of proportion are 

 reached, to corundum. 



On the other hand. Dr. J. H. Pratt,^ who has made a detailed study 

 of the North Carolina region, regards the corundum as an original 

 constituent of the peridotite — as having been held in solution in the 

 molten magma at the time of its intrusion into the country rock, 

 and having been one of the first minerals to crystallize on its cooling. 

 This view is most in accord with recent synthetic work done by Moro- 

 zewicz and others. 



Pirsson, who has described^ the occurrence of sapphires in a basic 

 eruptive rock from Togo Gulch, Montana, regards them as of pyro- 

 genetic origin — that is, they result from the direct crystallization of the 

 oxide, but which has been derived from aluminous material dissolved 

 from shales by the molten rock during its intrusion. The sharp out- 

 lines of the crystals in the granite from Gallatin County, Montana 

 (Specimen No. 83838, U.S.N.M.), is also indicative of a direct crystalli- 

 zation from a molten magma containing an excess of aluminum. A like 

 origin must also be recognized for the Canadian mineral, and a part 

 at least of that of India. 



Emery. — ^The rock emery takes its name from Cape Emeri.^ on the 

 island of Naxos, where it occurs in great abundance. Mineralogically 

 it has been regarded by various authorities as either a mechanical 

 admixture of corundum and magnetic iron ore or as simply a massive 

 iron spinel — hercynite. So far as the Naxos emery is concerned, the 

 first view is undoubtedly correct. Physically emery is a massive, 

 nearly opaque, dark gray to blue-black or black material, with a specific 

 gravity of 4 and hardness of 8, Dana's scale, breaking with a tolerably 

 regular fracture, and always more or less magnetic. 



Chemically the material is quite variable in composition, a fact 

 which gives support to the opinions of those who hold it to be a mixture 

 rather than a true chemical compound. Below are the results of 



^American Journal of Science, VI, 1898, pp. 49-65. 

 ^Idem, IV, 1897, p. 421. 



