216 



REPOBT OF NATIONAL MUSEUM, 1899. 



variety of chalcedony. The true onyx is similar to agate, except that 

 the bands or layers of different colors lie in even planes. Jasper is a 

 ferruginous, opaque chalcedony, sometimes used for ornamental pur- 

 poses. Opal is an amorphous form of silica, containing somewhat 

 variable amounts of water. 



Quartz occurs as an essential constituent of granite, gneiss, mica 

 schist, quartz porphyry, and liparite, and also as a secondary constitu- 

 ent in the form of veins, filling joints and cavities in rocks of all kinds 

 and all ages. 



Uses. — The finer clear grades of quartz are used to some extent for 

 spectacle lenses and optical work, as well as in cheap jewelry (Specimen 

 No. 11893, U.S.N.M.). Its main value is, however, for abrading pur- 

 poses, either as quartz sand or as sandpaper (Series Nos. 55877-56884, 

 U.S.N.M.), and in the manufactvire of pottery (Specimens Nos. 62123, 

 63035-63038, U.S.N.M.). For abrading purposes it is crushed and 

 bolted, like emery and corundum, and brings a price barely sufficient 

 to cover cost of handling and transportation. Pure quartz sand is also 

 of value for glass making (Specimens Nos. 53188, 60683, 63128, 63123, 

 63122, U.S.N.M.), and ground quartz to some extent as a "filler" in 

 paints (Specimen No. 63119, U.S.N.M.), and as a scouring material in 

 soaps. The following analyses show the composition of some glass 

 sands from (I) Clearfield and (II) Lewistown, Pennsylvania: 



Flint is a chalcedonic variety of silica found in irregular nodular 

 forms in beds of Cretaceous chalk. These nodules break with a con- 

 choidal fracture and interiorly are brownish to black in color (Speci- 

 men No. 62120, U.S.N.M.). By the aboriginal races the flints were 

 utilized for the manufacture of knives and general cutting imple- 

 ments. Later they were used in the manufacture of gun flints and 

 the "flint and steel" for producing fire. At present they are used 

 to some extent in the manufacture of porcelain, being calcined (Speci- 

 men No. 62061, U.S.N.M.) and ground (Specimen No. 62122, U.S.N.M.) 

 to mix with the clay and give bod}^ to the ware. In this countr}' the 

 same purpose is accomplished by the use of quartz. Small round 

 nodules of flint from Dieppe, France, are said to be used in the Tren- 

 ton (New Jersey) pottery works for grinding clay by being placed in 

 revolving vats of water and kaolin. All the flint now used in this 

 countr}^ is imported either as ballast or as an accidental constituent 

 of chalk. 



