MINERALOGY; WITH A CLASSIFICATION OF SILICATES. 81 



sphalerite, greeuoekite aud haiierite ; 2. Pi'oustoids, — corresponding to bournonoids, aud 

 ombraciug proustite and other red silver ores, cetrahedrite, livingstonite, dufreuoysite, 

 biunite, etc. It is worthy of notice that while siilphid of mercury, in the forms of meta- 

 cinnabar and cinnabar, appears in both suborders of the Metallates, the sulphid of anti- 

 mony is also represented among the Spathometallates by the well-known red and generally 

 nncrystalline kermès. The various forms of sulphur and of phosphorus, together with 

 vitreous selenium, will constitute a third tribe of the second suborder of Metallates. The 

 Spathometallates, as seen in their typical forms, sphalerite, vvurtzite, greenockite, cinnabar, 

 proustite, etc., serve, through the sul]ihoxydates, kermesite aud voltzite, aud through 

 sulphosilicates like helvite and danalite, to connect the order of Metallates with spathoid 

 Oxydâtes aud Silicates. 



§ 100. lu these various tribes the relations of hardness to condensation are not less 

 apparent than in Silicates and Oxydâtes. Dividing- the simplest atomic formula of the 

 complex Metallates l)y the number of atoms, we get, as the most convenient term for com- 

 parison, the mean weight of the elemental unit from which to deduce the volume, V. We 

 thus find for the pyritoids, pyrite and marcasite, values for V of 40 and 42; for linnieite, 

 4'4; for pyrrhotite and chalcopyrite, 50 and 5-3; and for alabandite, 5'4. The smaltoids, 

 niccolite and smaltite, give 44 and 51 ; the arsenopyritoids, cobaltite and gersdorfiite, 

 from 4 3 to 4 (i ; the thiogalenoids, for chalcocite, 'TO ; for stibnite, Y'4 ; for galenite, '7'9 ; and 

 for argentite, So. Of the sphaleroids, hauerite gives 5*7 ; sphalerite, 6'0 ; and other species, 

 vO-T^. The contrasts between the last two tribes and the preceding three, alike in their 

 hardness, and in their condensation, as shown in the different values of V, are apparent ; 

 and these are not less marked when the hard and dense arsenopyritoids are compared with 

 the chemically analogous, but softer, bournonoids and proustoids. Of the former of these, 

 euargite gives for V, 69, and bournonite, zinkenite and jamesonite, TT-T'S ; while of the 

 proustoids, miargyrite, proustite, pyrargyrite and polybasite give from 80 to 9'0, and 

 dufreuoysite, and tetrahedrite, from *7'2 to 8'3. By reason of the variations in the recorded 

 specific gTavities of most of the species compared, the values here given for V must be 

 regarded as but approximations to be corrected with the help of more exact determinations. 



§ 110. The native compounds of the haloid elements may be included under the order 

 Haloidate, with the four suborders of Fluor id, Chloricl, Bromid and lodid. Titanates, 

 niobates, tantalates, tungstates, molybdates, chromâtes, vanadates, antimonates, arsenates, 

 phosphates, nitrates, sulphates, borates, carbonates and oxalates constitute as many distinct 

 orders. Of these the soluble chlorids, sulphates, borates, carbonates, etc., belonging to the 

 salinoid type, form tribes under their respective orders, as Chlorosalinoid, Sulphatosali- 

 noid, Borosalinoid and Carbosalinoid. The native combustible carbons and hydrocarbon- 

 aceous bodies are included in a single order, which, from the fire-making property of 

 these may be aptly designated as the order of Pybicaustate8. This is divided into two 

 suborders : 1. Carbates, including the phylloid, graphite, and the adamantoid, diamond, 

 representing two tribes ; and 2. Carbhi/drates, which may be conveniently grouped in the 

 four tribes, Naphthoid, Asphaltoid, Resinoid and Anthracoid. 



§ 111. It is impossible to arrange in a single line the whole of the orders of natural 

 mineral species in such a manner as to show their affiliations. If, however, we place con- 

 secutively in a horizontal line the three orders of Metallate, Pyricaustate aud Haloidate, 

 the second of these will form the summit of a vertical column, in which, beneath the 



Sec. III., 1885. 11. 



