22 S THE GEOLOGIST. 



salts,* that the 0 (oxygen) in the E bases bears to the 0 in the R bases, 



which are combined together so frequently in silicate minerals. 



Taking Orthoclase again as an example (K Si^ + Ai Si^), here the O 

 in R : 0 in il : : 3 : 1. In Garnet, a mineral of most variable consti- 

 tution, this ratio is also constant at 1 : 1. Tn all minerals, however, 

 this relation cannot be accurately ascertained ; for in those containing 

 Iron or Manganese, the determining of the state of oxidation in which 

 they exist t in combination is a point attended with much difficulty. 

 This relation of the 0 in R : 0 in R we may call the Oxygen ratio. 



XXV. Mineral Species and their Classification. — The grouping 

 together of mineral species, on a scientific' system, is attended, as I 

 have already mentioned, with no small difficulty. In the descriptive 

 list of rock-minerals which follows, I have arranged them, as already 

 indicated, into (1) Elements and Binary-Compounds; (2) Carbonates, 

 Sulphates, and Borate ; and (3) Silicates. 



Elements; Binary-Compounds ; Carbonates, Sulphates, and Borate. 

 — I have already, in XtV and XVI., given a list of seventeen of these 

 substances as forming rock-minerals. I have now to extend this 

 number by the addition of five others, making in. all tioenty-two^ 

 These five are : Bitumen, the variety of carbon containing hydrogen ; 

 Opal, cr the hydrated variety of quartz; Brown-Hematite, or the 

 hydrous iicr-oxide of iron ; Magnetite, or magnetic-iron, a combination 

 of protoxi de and per-oxide of iron ; and Fyrrhotine, the magnetic 

 variety of Pyrite. 



XXVI. Silicates. — The leading silicated minerals naturally fall into 

 several groups or families — characterised by determinate' physical 

 characteristics and certain general chemical relations, — such as the 

 felspar group, the garnet and tourmaline group, the mica group, the 

 hornhlendic and augitic group, and the talc and olivine group. But the 

 connexion of these groups together in such a manner as to bring the 

 whole of the silicates into one general series is a very different matter, 

 and in a strict sense is not possible. But, if not strictly possible, 

 there docs yet seem to be in nature, with respect to these minerals, 



* " The vast mass of minerals are made up of compound silicates." See XIX. 

 p. 57. 



+ Tliat is, wlicther they exist as Fe, Mn, or J?e, Mn. 



