SALMON — ON ROCKS. 
57 
of lime, such as Wollastonite (Ca Si), are likewise unimportant. Of 
the simple silicates, those of Magnesia, such as Steatite (Mg Si) and 
Talc, arc tlie 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 
silicated 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-eartli, 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 coinpound 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 trifliug proportions as not to be worth our 
consideration here. Tlie silicates of Iron and Manganese are also 
important constituents of compound silicates ; 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 silicated minerals will give a general idea of the chemical 
relations of this important and complicated class of rock-con- 
stituents : — 
