THE EXTRUSIVE PROCESSES. 



613 



simply that of molten matter freezing. If lava be suddenly cooled, 

 the congelation is essentially the solidification of a melted substance. 

 The result is a glassy body, every part of which has essentially the 

 same composition that the Hquid had. Usually, however, even in this 

 case, the gases escape in part. If the cooling is slower, the various 

 substances in the mixture crystallize out into minerals in the order in 

 which they severally reach saturation. This involves the principle 



Fig. 470. — Terminal portion of a rough lava-flow, ^'aa. 



(Russell, U. S. Geol. Surv,^ 



Cinder Buttes, Idaho. 



that solubility is dependent on temperature, and that as the tempera- 

 ture sinks the degree of solubility declines, and the saturation-point 

 for some constituents of the solution is reached earlier than for others. 

 With sufficiently slow cooling, all the material will pass into the sohd 

 state by the crystallizing of the several minerals in succession. This 

 does not mean that two or m.ore minerals may not be forming at the same 

 time, for cr^^stals often interfere with each other's growth. It does, 

 however, involve the doctrine that some substances may complete their 



