76 THE SOIL. 



ccous Rocks ; fourth, Pudding-stone Rocks ; fifth, Lime 

 stone or Calcareous Rocks.* 



268. (1.) The Granitic Rocks get their name from 

 Granite, which is a hard rock composed of three minerals 

 called quartz, felspar and mica. Sienite is like granite, 

 but is composed of quartz, felspar, and hornblende ; and 

 Greenstone is composed of felspar and hornblende, with 

 out quartz. Traprock, another very hard rock which often 

 forms what seem to be natural walls, sometimes with 

 steps in their ends, is composed of felspar and horn 

 blende, with another mineral called augite. Gneiss and 

 Mica Slate, which look and are exceedingly like granite, 

 consist chiefly of mica and quartz, with felspar ; and 

 Porphyry is a very hard rock, made up almost entirely of 

 felspar. 



269. Granitic Rocks, including all those mentioned 

 above, are extremely hard, and are thought to be among 

 the oldest rocks. They, or the minerals of which they are 

 made up, are chiefly composed of 1, silex; 2, alumina; 

 3, lime; 4, potash; 5, magnesia; and 6, oxide of iron; 

 and, by their crumbling, or disintegration, form granitic 

 earths. 



270. Far the most abundant of these six is silex or 

 silica, which, as we have already said, is a metal-like sub 

 stance, silicon, chemically -united with oxygen. Though 

 it is not sour, it has other properties of an acid, acts as 

 one, and is called silicic acid; and the other five sub- 



* The teacher should, if possible, be furnished with a small collection of speci 

 mens of rocks and of the more important minerals found in them. ]&amp;gt;y means 

 of these his instructions may be made far more interesting and intelligible than 

 they possibly can be without. For perfect illustration of what is taught in this 

 chapter not more than twenty specimens will be required; and, by means of 

 such a collection, the pupils may easily be induced to make collections for them 

 selves, and to become acquainted with the names and qualities of all the rocks 

 in their neighborhood. 



