76 A TREATISE ON METAMORPHISM. 



p. 98). The increase in the size of the crystals, lessening the surface 

 tension, may be considered as a transfer of potential into kinetic energy. 

 This passes into heat and is dispersed under the apparently general law of 

 the dissipation of energy. Why the tendency to the transformation of all 

 forms of energy into heat and the dissipation of heat should be a law of 

 nature it is not my purpose here to discuss. But such the law seems to 

 be, and in its application we carry the causal sequence as far as we are 

 now able. 



The growth of large individuals at the expense of small ones in ground 

 water is of the most profound significance in the metamorphism of rocks. 

 It is illustrated by the secondary enlargement of minerals and by the por- 

 phyritic crystals which frequently develop in schists and gneisses, such as 

 the porphyritic crystals of feldspar, hornblende, garnet, staurolite, etc. 

 (See pp. 643-644, 699-700.) 



The above principle in reference to the growth of large crystals at the 

 expense of small ones is very clearly applicable to the growth of segregations 

 of minerals of a certain kind as compared Avith smaller segregations. If, 

 for instance, at one place there be a mineral aggregate, this, so far as the 

 surface tension and the free surface of liquids are concerned, acts as a unit 

 and tends to draw to itself the material of smaller aggregates or of individual 

 mineral particles. For aggregates which do not have crystal boundaries 

 the form which would be assumed under ideal conditions is spherical. 

 This principle of the growth of large aggregates at the expense of small 

 ones is illustrated bjr chert nodules. (See pp. 816-818.) 



The quantity of a solid which can be dissolved in aqueous solutions 

 depends upon the compounds present, the pressure, and the temperature. 

 When the limit of solubility is reached the solution is said to be saturated. 



COMPOUNDS PRESENT. 



Theoretically all compounds are soluble to some extent in water. This 

 statement applies to all natural compounds; that is, the minerals of nature 

 are elements, oxides, or salts which are soluble in water. No substance is 

 wholly insoluble in the ground solutions, even at the ordinary temperatures 

 and pressures. This statement is illustrated by the solution of quartz and 

 the more refractory silicates at the surface." Under surface conditions 



a Hayes, 0. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol. 8, 

 1897, pp. 214-217. 



