SILICA. SILICIOUS ROCKS. 77 



stances mentioned above, are usually combined with it as 

 silicates of potash, silicates of alumina, &c. 



When found pure, it is called quartz or flint, and in 

 that state is used in the making of glass. It is the most 

 abundant solid constituent of the earth s crust, forming 

 about five-eighths of the substance of the most important 

 rocks. Agate, chalcedony and opal, which are hard and 

 almost precious stones, are nearly pure silica. Though 

 so very hard, it is rendered soluble, and is dissolved by 

 the action of the alcalies and their carbonates. 



271. Silica usually occurs as coarse or fine sand, and 

 enters very largely into the composition of the soil of all 

 granitic regions, such as that of the greater part of the 

 New England States. Pure silicious sand is seldom found. 

 It is commonly mixed largely with grains of sand formed 

 by the crumbling of the other ingredients of the rocks. 



272. (2.) Silicious Rocks or sand-stones are composed 

 of small grains of silex agglomerated or stuck together, 

 and of various colors, from white to red, according to the 

 proportion of oxide of iron which they contain. When 

 crumbled into loose sand they make the poorest possible 

 soil. 



A soil formed principally of the sands coming from 

 these two sources, is a loose, light, sandy soil, readily pen 

 etrated by water, but not retaining it long, and therefore 

 liable to be much affected by drought. It is easily culti 

 vated, but not fertile, especially when its principal 

 ingredient is coarse silicious sand. Its fertility and its 

 readiness to retain moisture and manures depend upon 

 its fineness and upon the due admixture of other ingre 

 dients of soil, clay and lime, to be spoken of presently. 



273. (3.) 01aty or Argillaceous Rocks are all inore or 



