COLLKCTIONS IN ECONOMK; GKOLOOY AND Ml". lA Li,ri{<; Y. 203 



(IKINDINC. AND i'OMSIlINC M A'l'i;!: I A I.S. 



ruder tins licad ai«' iiiclmlrtl the naturally (mciui in;; inatcrials wliidi 

 art" iist'd for cutting;, ahraidin^', ;,Miiiilin;;, and jjolisliin;,' in all its forms, 

 such as ^Mindin^ to remove sui>erflu<»n.s material, to ;,'et a proper form 

 oi- to j;ive an ed^e, and polisliiu;; to t^ive a suitable linisli. I-'or this 

 jmrpose it is essential that the material shall he hard, and have a sharp 

 flitting edge when hroUen into small pieces. This cutting edge (let43r- 

 mines tlie value of the material. These eoiulitioiis being mainly phys- 

 ical a number of otherwise widely <lilferent minerals are used, and 

 generally only certain varieties of a mineral. Thus, while quartz in the 

 form of compact crystals, whi(th break with a sharp edge, is a very 

 good material, yet quartz in the form of a tine sand of rounded particles 

 has almost no value. 



The principal minerals used for this purpose are corundum and its 

 impure form emery, several varieties of quartz and garnet, while other 

 minerals are occasionally used. Of these corundum is by far the best, 

 both on account of its hardness and sharp cutting edge. Some varie- 

 ties of glassy quartz have such shari) edges when broken that they are 

 very effective. Besides distinct minerals, several varieties of rocks are 

 used, such as pumice stone, whicih is a volcanic feldspathic rock, and 

 rotten stone, which is a very much decomposed rock. 



CoRrXDUM. 



(I) Rough crystals, with a littli' <lain()nritt'. (_'i)l>b County, Georgia. (56340.) 

 (•2) Massive, with daiiiourite. Lueas Miue, Uahun County, Georgia. (.'itJ^.'jG.) 

 ('^) Coarse sand. Carter's Mine, Madison County, North Carolina. (.'>r»:}ll.) 



The (leorgia lump corundum and the commercial sizes ground from 

 it aie represented by twelve specimens from the Union Stone Com- 

 l>any, of Boston, Massachusetts. 



(I) Liinip ore. (55r«tf0.) 

 (•») Ground, size 18. (55876.) 

 (!{) (Jround, size 20. (.'JGH75.) 

 (4) (iround, size 24. (5,')y74.) 

 (.')) (iround, size :{(». (5.'>H7;{.) 

 (6) Ground, size 40. (5r»872.) 



(7) ( ironnd, size r>4. i.');"!^?!.) 



(H) (;round, size 60. (.^»5870.) 



(l>) Ground, size 70. (5.")869.) 



(10) Grounil, size 80. (55'?6"^.) 



(U) Ground, size 100. (.'.58«>7.) 



(12) Ground, size 120. (.'>58()6.) 



EMEKV. 



ICmery is an imixire and softer form ot' curnndiiiii. and i^ ot inferior 

 quality for grindin/r. 



(1) Cornndnni, magnetite, inargarito and chlorite, rough crystals. Sackett Mine, 

 Chester, Hampden County, Mas-sachusettM. Two Hpecimons. (4676.) 



(2) (iround, four grades ; separate<l from the Chester ores. (18186) An average 



