SIMPLE MINERALS. 183 



with the bases above named. The nitrate of potassa is the 

 nitre or salt-petre of commerce. The other nitrates are rare- 

 ly found in the rocks. Nitrate of soda is next to nitre in 

 importance. 



5. Phosphates. In these compounds, the acid is the phos- 

 phoric, and the most abundant sahs are the phosphates of lime, 

 found in most rocks ; phosphates of iron, soda, potassa, etc. 



6. Muriates. The muriatic acid forms but few com- 

 pounds, which exists in any considerable quantities in the 

 rocks. Common salt, when dissolved in water, has been re- 

 garded as a muriate of soda. It is found in sea water, and 

 widely diffused in certain geological formations, (the new red 

 sandstone,) but most writers regard it as a chloride of sodium, 

 a compound of chlorine and sodium. 



The compound bodies, Vv'hich have been enumerated, are, 

 with the exception of silicic acid and carbonate, sulphate 

 and phosphate of lime, rarely found in the rocks in a pure or 

 separate state. They are variously combined, and form the 

 natural substances, called the simple minerals ; and the sim- 

 ple minerals, united mechanically, and not chemically, form 

 the rocks. 



In order to understand the character of the rocks and the 

 soil, it will be necessary to describe these compounds, as 

 they actually exist in nature. 



Sect. 3. Simple Minerals which enter into the composition 

 of the Rocks. 



Of the three or four hundred species of simple minerals, 

 only seven or eight form the great mass of the rocky strata of 

 the globe. These are quartz, mica, feldspar, hornblende, 

 talc, serpentine, calcareous spar, or limestone and pyrites. 



1. Quartz is nearly pure silicic acid. It exists under a 

 a great variety of forms, and presents different appearances. 

 The purest variety is rock crystal, which is limpid and 



