68 OUR PHYSICAL WORLD 



England, New York, the Virginias, and Carolinas, and down into 

 Georgia. They are similarly found in the Rocky Mountain 

 regions and in the Ozarks. 



Because of their very wide distribution, the granites have 

 played an important part as the source of soils. The feldspar 

 which they contain weathers readily and, as a result of its weath- 

 ering, changes to kaolin, which, when permeated with such impuri- 

 ties as the oxides of iron, gives our common clays. The quartz 

 is, of course, more resistent to the weather but is sorted out by 

 the water, is more or less weatherworn, and is deposited as beds 

 of sand. 



In the syenite-trachyte group only two rocks are given. The 

 syenite is the coarsely crystalline or plu tonic member; the 

 trachyte, the finely crystalline or volcanic member. The sye- 

 nites are not very common. They consist of orthoclase chiefly, 

 though other minerals, like mica, hornblende, pyroxene, may be 

 present. The quartz is either absent or present in such small 

 quantities as to be a negligible constituent. Trachyte is a very 

 fine-grained rock of similar constitution. It can usually be 

 recognized, in spite of the fact that the constituent minerals are 

 in such small particles that they are distinguished with difficulty, 

 by its light color and light weight. 



The diorite-andesite group includes diorite, sufficiently 

 coarsely crystalline so that the constituent minerals may be dis- 

 tinguished, and andesite, very finely crystalline. In the diorite- 

 andesite group, the feldspar present is of the dark variety, plagio- 

 clase feldspar. Hornblende is also present and equals or exceeds in 

 its amount the feldspar. Quartz is also usually present, and there 

 may be other accessory minerals. It is evident from the composi- 

 tion that the diorites grade into the granites, on the one hand, 

 and it will be seen that they grade into the gabbros, on the other. 

 When the constituent minerals are present in very tiny grains 

 so that it is quite impossible to make out the individual com- 

 ponents, the rock is known as an andesite. When the constituent 

 minerals occur, any one of them in large crystals, while the rest 



