THE NONMETALLIC MINERALS. 



467 



Hiie-oniiiH'd srliistoso dark-gray rocks, .soinetimes of a light chocolate 

 color on a freshly fractured surface. The microscope shows them to 

 consist of a compact and slightly schistose aggregate of quartz and 

 mica in which arc frcijucntly included very abundant small octahedral 

 crystals of magnetic iron and sometimes garnets. (See Plate 28.) So 

 abundant are these magnetite granules in some of these rocks, espe- 

 I'ially tiiosc of Grafton, New Hampshire, as to constitute an important 

 feature, and it is doubtless very largely to them that the stone owes its 

 excellent abrasive qualities. Magnetite, it will be remem})ered, has a 

 hardness of about 0.5 of the scale, and constitutes a very considerable 

 proportion of the ordinary emery of commerce. We have here, then, 

 what is almost a natural equivalent of the artificial emery stone, the 

 compact groundmass of quartz and mica serving as a l)inding material 

 for the magnetite grains wdiile they perform their work in wearing 

 away the implement being ground. A part of the abrading quality of 

 these stones is, however, due to the abundant quartz and mica scales 

 and their peculiar arrangement in relation to one another. 



The well-known Water of Ayr, Scotch hone, or snake stone, as it is 

 \ariously called, is also a very compact schist. It is said to come from 

 Dalmour, in Ayrshire, Scotland. (Specimens Nos. 38931, 38937, 38946, 

 54146, U.S.N.M.) 



The name novaculite is applied to a very fine-grained and compact 

 rock consisting almost wholly of chalcedonic silica, and which, owing 

 to the fineness of its grit, is used only in the finer kinds of work, as in 

 sharpening razors, knives, and the tools of engravers, carpenters, and 

 other artisans. The true novaculites are, so far as the writer is aware, 

 at present quarried in America onl}^ in Montgomery, Saline, Hot 

 Springs, and Garland counties, in Arkansas, and are known commer- 

 cially as the Washita (or Ouachita, as the name is properly spelled) 

 (Specimens Nos. 38955, 38906, 38969, 38977, 72900, etc., U.S.N.M.), 

 and Arkansas stones (Specimens Nos. 38954, 38971, U.S.N.M.). Both 

 varieties are nearly pure silica, the Ouachita being often of a yellowish 

 or rusty red tint (Specimen No. 72900, U.S.N.M.), and the Arkansas 

 of a pure snow whiteness, the latter variety being also the hardest, 

 most compact, and highest priced. The analyses given below show 

 the average composition of the two varieties: -- 



