COLLECri(»N8 IN KCONOMIC GEOLOlJV ANi> MK TAI,Li;i{(, V. J 1 7 



(11) Pottery iliiy. West Chester, Chostor County. PoaiKsylvani.i (WoT't. ) 



(12) Pottt-ry cluy. Dt-Iawiini. ('JltTld.) 



(l:{) I'oUi-ry i-lav. Coliiiiiliin, Rirlilaml County, Soiitli Can>liiia. ('iUT'-JH.) 



(14) i'ottcry olay. ("olunihia, liirlilaud County, Soutli Carolina. (IWIWI!.) 



(l.'i) Pottery clay. Au;iu.sta, Lincoln County. Oeorjiia. ('J'J7-.i7.) 



(If)) Pottery clay. Lawrence County, IiKliana. (2'.»7U.) 



(17) Pottery clay. Wa<'o, Madison County, Kentucky. (:{:{I»7r>. ) 



(IH) Pottery clay. Jeflersou County, Mi.sHouri. (17^1').) 



(I'J) Pipeclay. Westport, Ivssex County, New York. (202>Ht.) 



I" III': (' A i: r, () n c. 1:0 r r. 



The aerie.s of iiiiturally occurring' inatcrials in wliidi caihon forms 

 tlu' cliiof constituent contain.s sever.il classes of widely dilleriii;,' iiia- 

 teiials of iniicli economic importance. Taken as a whole, this <,'r(>ni) is 

 most naturally and scientifically classified by the increasin^r amounts 

 of volatile hydiHxarhon.s, which the materials contain or evolve upon 

 being heated. Besides this increasing amount of gaseous constituents, 

 there is also a gradual change in physical characters, beginning witli 

 diamond, the hardest of known substances, thence through an iuiireas- 

 ing softness to liipiids, and eniling with a gas. This group may bo 

 naturally divided into the pure carbon, coal, and bitumen series. 



Pure Carbon Series. 



DIAMOND. 



Composition: C. Pure Ofirbon. 



It crystallizes in the isometric system, generally with an octahedral 

 habit. The faces are usually curved, and twins are (•(►mmon. it has a 

 very perfect octahedral <-leavage. Its hardness is 10, being the hardest 

 substance known. Its specific gravity is 3.5. It is usually white and 

 colorless, but may be tinged many colors. Its luster is characteristic. 



The diamoJid is found mostly in sand, gravel, or clay of recent allu- 

 vial formation. The i>rincii)al localities are in India, South Africa, 

 and Brazil. The principal value of the diamond is as a gem, but small 

 amounts are used for cutting hard material.s, and as the cutting edge of 

 (Irills, especially rock drills. 



(1) AsinjjIeLKliau dianionil is uliown her.^ ; lurtlier illustrations can he seen in the 

 geniseriuHof the inin<>ral department. (.V.ni7.'>. i 



GRAPHITE. (PLITMHAGO, HLACK LEAK, <'A KltlKHI" (>F IKON.) 



Compo.sition: i\ Pure carl.on. olten eonlaining a small ainumil of 

 iuipuiity. 



It crystallizes m the hexagonal .sysii-in in tabular lornis with Itasal 

 cleavage. Crystals are rare. It generally occurs in foliated or grann 

 lar masses. Its hardness is 1 to J, and specific! gravity 2 to L'.'J. It is 

 iron-black to .steel -ray in color. It is .sectile and .soils everything it 



