114 A TREATISE ON METAMORPHISM. 



various ions there are also various crystalline minerals, the moment that 

 the solution becomes supersaturated with reference to several ions which 

 may unite to produce one of the solids present, this union will take place, 

 the material will be precipitated upon the minerals of that kind, and thus 

 they will grow. 



This process of mineral growth applies alike to minerals in magmas and 

 to minerals in sedimentary rocks. If, for instance, in a magma j;>lagioclase 

 and pyroxene individuals once begin to form, they may grow to larg*e size 

 and produce a gabbro. In a sedimentary rock in which quartz and feld- 

 spar particles are present and the solutions are of a kind which furnish 

 constituents for their growth, these particles are likely to be enlarged. 



Supersaturation, and consequently precipitation, may result in various 

 ways, of which the following are the more important: (1) Precipitation 

 by change of pressure, (2) precipitation by change of temperature, (3) 

 precipitation by reactions between aqueous solutions, (4) precipitation, 

 by reactions between aqueous solutions and gases, and (5) precipitation by 

 reactions between solutions and solids. 



PRECIPITATU»' BY CHAXCJE OF PRESSURE. 



Change of pressure may result in supersaturation, and therefore in 

 precipitation. Where the volume of the solution is less than that of the 

 solvent and substance dissolved, decrease of pressure is favorable to 

 precipitation. Where the volume of the solution is greater than that of 

 the solvent and substance dissolved, increase of pressure is favorable to 

 precipitation. The volume relations are opposite in the cases of the 

 crystallization of minerals from solutions of ground water and the crystal- 

 lization of minerals from magmas. In the case of substances dissolved in 

 ground solutions the volumes of the solutions are commonly less than those 

 of the solvent and the substances dissolved; therefore decrease of pressure 

 is favorable to precipitation. But in the case of crystallization from 

 magmas the volume of the solution is greater than that of the crystallized 

 minerals; therefore pressure is favorable to crystallization. 



In another connection it is suggested that under certain conditions water 

 and magma are miscible in all proportions. (See .Chapter VIII, p. 723.) 

 In other words, there is every gradation from water containing compounds 

 in solution to magmas containing subordinate amounts of water. If this 



