186 GEOLOGY AXD CHEMISTRY OF SOILS. 



of carbonate of lime ; from 1 to 40 per cent, of insoluble mat- 

 ter, probably silicate of alumina ; in some cases, 4 per cent, 

 of oxide of iron, from 0. to 40 per cent, of magnesia. The 

 limestones of Massachusetts, according to Prof Hitchcock's 

 analysis, contain from 44.8 to 99.6 per cent, of carbonate of 

 lime ; from to 40 of carbonate of magnesia in some species ; 

 from to 8 per cent, of carbonate of iron ; and from 0.4 to 

 61.6 of silicate of alumina. The sulphate of lime or plaster, 

 with the phosphate, may also be included in this group, as in 

 some cases entering into the composition of the rocks. 



Pyrites, or iron pyrites, is a bisulphuret of iron, and exists 

 extensively in primitive rocks, but in much less quantities 

 than the preceding minerals. It resembles gold, and is often 

 taken for that substance ; hence it has been called ybo/'s gold. 



It will be seen that silex or silicic acid is the most abun- 

 dant ingredient in those simple minerals above enumerated, 

 and alumina the next. They are mostly silicates and are di- 

 vided by Dana into three classes. 



1. Silicate of alumina and potash form feldspar and mica. 



2. Silicate of alumina and lime, with magnesia, form horn- 

 blende. 



3. Silicate of alumina and magnesia form serpentine and 

 talc, and silicic acid forms quartz. 



Sect. 4. Composition of the Rocks. 

 Rocks are composed of the simple minerals. In some cases, 

 the minerals may be seen in separate portions, united me- 

 chanically, as in granite. In other cases they are so inter- 

 mingled as to conceal their distinct characters, as in green- 

 stone. Rocks are divided by geologists, according to their 

 supposed origin, into two separate classes. 1. Igneous 

 rocks, or those which appear to have been fused by fire. 

 2. Aqueous rock, or such as appear to have been deposited 

 from water, or which have resulted from the decay and wear- 

 ing down of the first class. The igneous rocks are highly 



