228 
THE GEOLOGIST. 
salts,* that the 0 (oxygen) in the Ji bases bears to the 0 iu the R bases, 
— which are combined together so frequently in silicate minerals. 
Taking Orihoclase again as an example (K Si^ + Al Si^), here the O 
in B : 0 iu R : : 3 : 1, In Garnet, a mineral of most variable consti- 
tution, this ratio is also constant at 1:1. In 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 B : 0 in R we may call the Oxijgen 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, Sulj^hates, and Borate. 
— I have already, in XIV 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 tiventy-tivo. 
These five are : Bitumen, the variety of carbon containing hydrogen ; 
Opal, or the hydrated variety of quartz ; Brown-Hematite, or the 
liydroii - [)er-oxide of iron ; Magnetite, or magnetic-iron, a combination 
of proto.\lde and per-oxide of iron ; and Byrrhotine, the magnetic 
variety of Pyi'ite. 
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 angitic 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 does yet seem to be iu nature, with respect to these minerals, 
* " The vast mass of minerals are made up of compound silicates." See XIX. 
p. 57. 
f That is, whether they exist as f c, Mii, or t'c, Mii. 
\ 
