80 THE SOIL. 



mechanical properties it is a medium between clay and 

 sand. 



277. (5.) Pudding-stone Rocks, sometimes called gray- 

 wacke, are made up of materials formed by the mixture 

 of a great variety of other rocks, -which seem to have 

 been brought together, in very ancient times, by the action 

 of floods or streams of water. They have their name 

 from their resemblance to plum-pudding, the ingredients 

 being of every variety of lime-stone, clay-slate, and porphy 

 ry, greenstone, trap, and every other form of granitic rocks. 

 They are often of a very coarse texture, made up of 

 pieces of stone of every size, sometimes weighing hun 

 dreds of pounds, and sometimes of so fine a texture as to 

 resemble slate. 



The materials are held together by a natural mortar of 

 lime or of rust of iron, or by mere contact. When com 

 pletely reduced to dust, these rocks make a rich soil, from 

 its containing all the mineral materials, intimately mixed, 

 which are necessary to the fertility of soil. 



278. All these rocks, differing in hardness and in other 

 properties, and forming, perhaps, at first, the surface of 

 the earth, have, in process of time, been crumbled, and 

 then, or before, transported to various distances. 



The sand, coarse or fine, formed by the crumbling of 

 the granitic rocks, sand-stones, and pudding-stones, con 

 tain the six substances enumerated, 209. The slate &quot;rocks 

 form clay; and the chalks and other calcareous rocks, 

 lime. Altogether they furnish all the mineral materials 

 which enter into the structure of plants. 



279. How have these rocks been changed into soil? 

 Chiefly by the action of heat, of water, and of cold. The 

 sun s heat warms and expands all the rocks upon which 

 it falls. While they are in this state, the rain, descending, 



