GUIDE TO THl: STUDY OP THl: COLLF£TIO\S IN THE 

 SECTION OF AlTTJliD GEOLOGY. 



THE NONMETALLIC MINERALS. 



By George P. Merrill, 



Curator, Dlri>ilo)) of Phy^vcal (tnd Chemical Geology and Head Curator of the Department. 



I. ELEMENTS. 

 1. Carbon. 



Tho numoroiis ooinpounds of which carbon forms tho chief constit- 

 uent are widely variable in their physical properties and origin. As 

 occurring in nature few of its nieuibers p >ssess a definite chemical 

 composition such as would constitute a true mineral sp(H'it\s, and they 

 must for the most part be looked upon as indefinite^ admixtures in 

 which carl)on, hydrogen, and oxygen play the more iniportint roles. 

 For present purposes the entire group may l)e l)est considered under 

 the heads of (1) The Pure Carbon series; (i^) Th(> Coal series, and (8) 

 The Bitumen series, the distinctions being based mainly on the gradu- 

 ally increasing amounts of volatile hydrocarbons, a change which is 

 accompanied by a variation in physical condition from the hardest of 

 known su))stances through plastic and liquid to gaseous forms. Here 

 will be considered only the members of the pure carbon series, the 

 others being discussed under the head of hydrocarl)()ii compounds. 



Diamond. — This mineral crystallizes in the isometric syst<Mn, with 

 a tendency toward octahedral forms, the crystals showing curved and 

 striated surfaces, (Specimen No. 53558, U.S.N.M.) The hardness is 

 grejit, 1(> of Dana's scale; the specific gravity varies from 3.1 in the 

 carl)onados to 3.5 in good clear crystals. The luster is adamantine; 

 the colors, white or colorless, through yellow, red, orange, green, 

 brown to black. The transparent and highly refractive forms are of 

 value as gems, and can best be discussed in works upon this subject. 

 We have to do here rather with the rough, confused crystalline aggre- 

 gates or rounded forms, transluc(Mit to opaijue, which, though of 

 no value as gems, are of the greatest utilit}' in the arts. To such 



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