384 GEOLOGY 



substances. Chief among them, as already stated, are silicates of 

 aluminum, potassium, sodium, calcium, magnesium, and iron, 

 with minor ingredients of nearly all known substances. The 

 stages at which saturation for certain compounds is reached vary 

 somewhat widely. 



The old idea that lava is melted rock, is not, however, to be 

 abandoned wholly. Lava sometimes solidifies much as water 

 freezes. Thus when lava is suddenly cooled, the congelation is 

 essentially the solidification of a melted substance. The result 

 is a glass, every part of which has essentially the same composition 

 that the liquid had. Even in this case, however, some of the gases 

 escape. If the cooling is slower, the various substances in the mix- 

 ture crystallize out into minerals in the order in which they sev- 

 erally reach saturation. This involves the principle that solubility 

 is dependent on temperature, and that as the temperature sinks 

 the degree of solubility declines, and the saturation-point for some 

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

 With sufficiently slow cooling, all the material passes into the solid 

 state by the crystallizing of the several minerals in succession. 

 This does not mean that two or more minerals may not be forming 

 at the same time, but it does mean that some minerals may be 

 crystallized out while the surrounding material is still fluid. In 

 most igneous rocks, nearly perfect crystals of certain minerals are 

 common, while other minerals, crystallizing later, adapt them- 

 selves to the space left. This conception is supported by the fact 

 that lavas, while still in the fluid condition, often contain well- 

 formed crystals, and these crystals sometimes make up a consid- 

 erable part of the flowing mass, very much as water in certain 

 conditions may be filled with crystals of ice. 



The temperature of lava. Accurate determinations of the tem- 

 peratures in the centers of lava-columns, where they have been 

 least reduced by contact with the rock-walls, have not been made; 

 but it is clear from the white heat of some lavas that their temper- 

 atures are often appreciably above the melting-point. This is also 

 a necessary inference from the length of time lavas remain fluid. 

 notwithstanding the great surface of contact of the column in its 

 miles of ascent, the conversion into steam of the water in the rock 



