SALMON— ON ROCKS. 



5T 



of lime, such as WoUastonite (Ca Si), are likewise uuimportant. Of 

 the simple silicates, those of Magnesia, such as Steatite (Mg Si) and 

 Talc, are the only ones -which occur in abundance, and form important 

 constituents of large rock-masses. Silicates of the alcalies do not 

 occur uncombined as minerals, although combined with the earthy 

 bases they constitute a large part of the most frequently occurring 

 silieated minerals : this arises from their great solubility. Simple 

 silicates of the oxides of iron and manganese are, however, met with, 

 although not in any abundance. Persilicate of iron has a light ochre 

 colour, but the protosilicate is green. The mineral Glauconite, or 

 green-earth, is essentially a silicate of protoxide of iron ; granules of 

 it are frequently abundant in chalk, tertiary, and other strata. Simple 

 silicates of manganese also occur, but in trifling quantities ; most of 

 them are hydrated. 



XIX. Compound Silicates. — The vast mass of minerals are made up 

 of compound silicates. Among the bases which are found most fre- 

 quently and abundantly in this class of minerals, Alumina is by far 

 the most important. In a large number of them — and these among 

 the most important — it is the prevalent base, and of the whole 

 number of compound silicates there are comparatively few in which 

 it is wholly absent. Next to Alumina, the alcaline earths. Lime and 

 Magnesia, and the alcalies, Potash and Soda, are the most abundant 

 bases in this class of silicates. Lithia is comparatively rare, and 

 Baryta occurs in such trifling proportions as not to be worth our 

 consideration here. The silicates of Iron and Manganese are also 

 important constituents of compound silicates j they are usually the 

 protoxides of these metals. The compound silicates are consequently 

 made up of various combinations of the silicates of 



1. Alumina. 4. Potash. 7. Proto (per) Oxide Iron. 



2. Lime. 5. Soda. 8. Proto (per) Oxide Manganese. 



3. Magnesia. 6. Lithia. 



XX. The following synopsis of the most important simple and 

 compound silieated minerals will give a general idea of the chemical 

 relations of this important and complicated class of rock-con- 

 stituents : — 



