Building Materials, Their Use and Origin 409 



up the level prospect of the desert lands of the West are often 

 granite. Granite also cuts across other rock formations and 

 is then found as veins. In the New England region granite, 

 generally of the gray color, occurs over a large area. The 

 abundance, durability, and beauty of this rock account for 

 its wide use. 



Granite is very durable ; but it eventually decays, although 

 the weathering process is exceedingly slow. The felspar may 

 be broken up into very minute particles which form clay. The 

 rest of the rock may form sand, though the mica and hornblende 

 may finally be dissolved away by water. Or the felspar, mica, 

 and quartz may be broken up into exceedingly fine particles 

 to form mud. Granite is thus seen to be the source of the 

 materials which often make up sandrock and shale. 



Formation of granite. When you first examine a piece of 

 granite you may suppose that, like sandstone, it is made up of 

 small particles. But this is not the case. The particles are 

 crystals, as examination will show, and were formed in place. 

 They also show the effects of the heat to which they were sub- 

 jected and of the manner in which they were cooled. Granite 

 is formed from the molten mass that cooled gradually and con- 

 solidated beneath the earth's surface. In the lavas which 

 came molten to the surface are found crystals which were 

 formed before the lavas were poured forth upon the earth. In 

 rock of this kind the felspar crystals are conspicuous ; but the 

 rest of the rock is fine-grained, and the crystals that have had 

 time to form under the conditions of quick cooling at the sur- 

 face are very small. This kind of rock is described as por- 

 phyritic (page 408). In the rock which formed beneath the sur- 

 face of the earth, the crystals are complete individuals and 

 conspicuous, and they give to it its characteristic features. 

 This condition of the rock structure indicates that it cooled 

 gradually and slowly enough to allow crystallization to continue 

 without disturbance. This is the rock commonly called granite. 

 Because such rock is formed from molten materials, it is gen- 



