VOLCANOES AND IGNEOUS INTRUSIONS 



301 



mutual solution of mineral matter in mineral matter. Gases as well 

 as mineral matter enter into the solution. 



This can best be illustrated by a well-known experiment. If 

 crystals of snow, salt, and sugar are mixed together and compacted 

 at a low temperature, an artificial rock will be formed in which the 

 constituents can be recognized. If the temperature of this solid is 

 now raised to about 3 2° F. the mass will become a liquid, even though 

 the melting points of salt and sugar are very much higher. In this 

 case, a rise in temperature sufficient to melt but one of the constituents 

 is necessary, since this one is 

 then capable of dissolving the 

 others. If the temperature 

 of such a solution is again 

 lowered, the salt and sugar 

 will not crystallize out until 

 they are forced to take the 

 solid form by the crystalliza- 

 tion (freezing) of the water. 

 It is evident that both in the 

 process of solution and in that 

 of crystallization the important 

 factor is solubility, and that a 

 temperature merely sufficient 

 to melt one of the constituents 

 is necessary. 



That lava should be con- 

 sidered as a solution of vari- 

 ous minerals is evident when 

 cooled lavas are examined. If 

 lavas were simply molten rocks in which the minerals had melted 

 according to their fusibility, we should find that upon cooling the 

 least fusible mineral would crystallize out first, then the others in 

 the order of their fusibility. Such, however, is not always the case ; 

 often the least fusible mineral is the last to take the solid form. This 

 is due to the fact that the liquid mass is a solution in which the 

 various minerals assume the liquid state, and upon cooling, the solid 

 state, depending upon their solubility more than upon their fusibility, 

 the least soluble rather than the most infusible crystallizing first. 



When a lava cools very slowly, as is usually the case when it is 

 intruded beneath the surface, the molecules of which it is composed 



294. — Scoriaceous lava. 

 National Museum.) 



(U. S. 



