122 A TREATISE ON METAMORPHISM. 



mineral species will prevent considerable supersaturation of the solution, so 

 far as the compounds of that species are concerned. The result is that if 

 .there be materials in a solution which can unite to produce mineral species 

 which are present they will do so. In this way the minerals control or 

 guide to a considerable extent the character of the solids which are deposited, 

 since when a certain mineral is absent, before that mineral can begin to be 

 precipitated supersaturation must occur with reference to the - chemical 

 combination which composes it. 



Therefore the mineral species which are present in a solution have 

 an advantage over other kinds of minerals which are absent. To a less 

 degree, minerals which are abundantly present have an advantage over 

 those which are sparse. To illustrate, if quartz be present and the solu- 

 tions contain ions of silica, it will be apt to abstract the silica from 

 the solutions the moment supersaturation occurs. In the same way, if 

 feldspar be present and there are ions of sodium, calcium, aluminum, and 

 silica in proper proportions, these are likely to be grouped together to 

 produce feldspar. Moreover, it appears to be the case that the feldspar 

 may so nearly control that a closely analogous feldspar is produced, and 

 twinning and other phenomena characteristic of the original grains be 

 continued in the secondary growth. The same statements applv to 

 hornblende, tourmaline, calcite, and, in fact, to all minerals in which a 

 secondary growth has been noted. Of course, in a rock in which there 

 are present a large number of mineral particles, the particular mineral 

 which is formed will depend upon the various ions in the solution, their 

 relative proportions, and the relative insolubility of the salts. For instance, 

 tourmaline can not form unless the boric acid ions are present; horn- 

 blende can not be produced unless there are in the solution all the bases 

 demanded by that mineral in sufficient abundance. Thus the particular 

 mineral which forms depends upon a complicated adjustment of the mineral 

 particles present, the ions present in the solution, their relative proportion, 

 and the solubility of the mineral particles. 



In the above chemical principle lies a partial explanation of the strange 

 fact that minerals are so firmly cemented by material like the dominant 

 original mineral. In quartzose sandstone the chief cement is silica; in feld- 

 spathic sandstone the chief cement is likely to be feldspar; in strongly horn- 

 blendic rocks one of the chief cements is hornblende, and so on. Another 



