26 PHYSICAL GEOLOGY 



have the structure and luster of glass, and so are called glassy 

 rocks (Fig. 9). When composed of very small crystals, the 

 rock may have a dense, stony appearance, rather than a glassy 

 luster. If, after certain minerals have crystallized out, the 

 still liquid mass in which they float be cooled suddenly, a 

 rock may result which is partly glassy or stony and partly 

 crystalline (Fig. 8). 



The rate of cooling, then, is a chief factor in the crystalliza- 

 tion of igneous rocks. The rate is influenced by several con- 

 ditions, of which the following are chief, (a) Large bodies 

 of lava cool less rapidly than small ones. (6) Masses deep 

 within the earth's crust cool more slowly than those at or near 

 the surface, (c) The rate at which a body of lava cools is 

 affected also by its shape. Thus a globular mass containing 

 the same quantity as a thin sheet would cool far less rapidly. 

 (What combination of these conditions would most favor the 

 formation of coarse-grained rocks? Fine-grained?) 



(2) The more fluid the lava, the easier and the farther may 

 the molecules move, and the larger, other things being equal, 

 may crystals grow. The mobility of the lava is determined 

 largely by its temperature and composition, but partly by 

 the amount of water vapor it contains, and to less extent by the 

 presence of other volatile substances, such as carbon dioxide 

 and fluorine. In addition to hindering the lava from becom- 

 ing stiffly viscous, (a) the waters, etc., lower the temperature 

 at which solidification occurs and so prolong the period of 

 crystal growth, and (6) some common minerals do not form 

 save in their presence. These substances, especially water, 

 influence the process of crystallization so greatly that they are 

 appropriately called mineralizers. That many lavas contain 

 large quantities of water vapor is familiarly illustrated by the 

 heavy rains which frequently attend volcanic eruptions, due 

 to the condensation of escaping steam. 



(3) The direct effect of pressure upon texture is probably 

 not great. Most rocks contract on becoming solid, and, 

 were other things equal, lava would therefore solidify more 



