THE NONMETALLIC MINERALS. 807 



H. Nelles Thompson. Asbestos Mining and Dressing at Thetford. 



The Journal of the Federated Canadian Mining Institute, 1897, II, ]>. 273. 

 See also the Canadian Mining Review , XVI, 1897, p. 126. 

 RomcRT H. Jones. Asbestos and Asbestic: Their Properties, Oceurrencr. antl Use. 

 London, 1897, pp. 368. 



4. (lAIiNET. 



The chemical compo.sitioii of the various; minerals of the garnet 

 group is somewhat variable, though all are essentially silicates of 

 alumina, lime, iron, or magnesia. The more common types are the 

 lime-alumina garnet grossularite^ and the iron alumina garnet alaman- 

 dite. Other varieties of value as minerals or as gems 2kve pyrope, spess- 

 artite., andradlU^ hredhergite^ and rrrarovUe. 



The ordinary form of the garnet is the regular 12 or 24 sided solid, 

 the dodecahedron and trapezodedron, as shown in Specimen No. 53241, 

 U.S.N.M., from Roxbury Falls, Connecticut. The color is dull red or 

 brown, though in the rarer forms yellow, green, and white. Hardness 

 from 6.5 to 7.5 of the scale. 



Oecnrrenee. — Garnets occur mainl}' in metamorphic siliceous rocks, 

 such as the mica schists and gneisses, and though sometimes found in 

 limestones and in eruptive rocks, are rarely sufficiently abundant to 

 be of economic importance. In the gneisses and schists, however, 

 the}^ not infrequently preponderate over every other constituent, 

 varying from sizes smaller than a pin's head to masses of 100 pounds 

 weight, or more. 



The most important garnet-producing regions of the United States 

 are Warren County, New York, and Delaware County, Pennsylvania. 

 At the lirst-named localit}", the garnets occur in laminated pockets 

 scattered through beds of a very compact hornblende feldspar rock, 

 the size of the pockets ranging from 5 or 6 inches in diameter to such as 

 will yield 1,000 pounds or more (Specimen No. 53228, U.S.N.M.). In 

 the Delaware County localities the garnets occur in aggregates of small 

 crystals in a quartzose gneiss^ (Specimens Nos. 53221, 66710, U.S.N. M.). 



One of the most noted garnet regions of the world is that near 

 Prague, Bohemia. According to G. F. Kunz,*^ the garnets of the 

 pyrope variety arc indigenous to an eruptive rock now changed to ser- 

 pentine, and the mineral is found ''loose in the soil or in the lower 

 part of the diluvium, or embedded in a serpentine rock. * * In min- 

 ing for garnets the earth is cut down in banks and only the lower layer 

 removed, and the garnets are separated b}^ washing. The earth is 

 first dry sifted and then washed in a small jig consisting of a sieve 

 moved back and forth in a tank of water.'' 



Ihen. — Aside from their use in the cheaper forms of jewelr}^ garnets 



'The Mineral Industry, V, 1896. 



^Transactions of the American Institute Mining Engineers, XXI, 1892, p. 241. 



