THE COMPOSITION OF THE EARTH 



25 



Factors influencing the physical character of igneous rocks. 

 The circumstances under which lavas solidify vary greatly, 

 and many factors influence the texture of igneous rocks. 

 Among the leading ones are (1) the rate at which the lava 

 cools, (2) the fluidity of the lava, and (3) the pressure under 

 which it consolidates. The texture is influenced also by (4) 

 the chemical composition of the lava. 



(1) Lava is liquid rock, a solution in which certain minerals 

 are dissolved in others. The high temperature of the lava 

 appears to make it possible for the minerals to form a mutual 

 solution. As lava 

 cools, the point of 

 saturation of some 

 mineral present is 

 reached, and it be- 

 gins to take the 

 solid form. The 

 molecules of this 

 mineral tend to 

 collect and arrange 

 themselves in reg- 

 ular order, build- 

 ing up crystals 

 having a definite 

 geometrical form. 



FIG. 9. Obsidian, or natural glass. Shows the 

 glassy luster and fracture. About % natural size. 

 (Photograph by Baker.) 



As cooling proceeds, the point of satu- 

 ration of other minerals is reached, and crystals of other 

 kinds begin to form. With continued cooling, the entire mass 

 may become crystalline. Lava is probably never so fluid as 

 water, and is commonly rather stiff (viscous). It clearly re- 

 quires some time for molecules scattered throughout such a 

 liquid to come together and form crystals. Slow, regular cool- 

 ing therefore favors the development of large crystals. 

 The resulting coarse-grained rocks are sometimes called 

 granitoids (granitelike rocks). If cooling is rapid, the mass 

 is likely to become solid before crystals have formed, or while 

 they are still very minute. In the former case the rocks may 



