MATERIALS OF THE EARTH 



31 



stones embrace the diorites, dolerites, some of the gabbros, and the basalts, 

 and may even extend to the peridotites and perhaps to others. Another 

 convenient name is trap, which may be used for any dark, heavy igneous 

 rock, such as basalt. The term basalt is sometimes used in much the same 

 way. 



Varieties of rock dependent upon conditions. From what has preceded, 

 it is clear that the chemical nature of the liquid magma determines the 

 mineralogical composition of the rock, if it is crystalline. But it may now be 

 pointed out that the same lava which made a plutonic granite, might have 

 made a porphyry, an obsidian, a pumice, or a tuff, under other conditions of 

 cooling and hardening. The same is true of other varieties of the phanerites. 



The Disruption of Igneous Rocks 



At the surface, igneous rocks are subject to mechanical dis- 

 ruption, and to chemical change which results in decay. 



Mechanical disruption. The great agent of mechanical dis- 

 ruption is change of temperature. Heating by day and cooling 

 by night produce some such change in rock as that effected by 

 the sudden heating of cold glass or the sudden cooling of hot glass. 



Fig. 13. Exfoliation of granite. Wichita Mountains, Okla. 



The heating of the surface of the rock by the sun expands it, and 

 since the outer part is heated and expanded more than that below, 

 a strain is set up between the superificial part and that below, 

 and this strain may be enough to break off the outer part. " Shell- 



