636 A TREATISE ON METAMORPHISM. 



and with this expectation analyses of waters of springs fully accord. With 

 the carbonates are also salts of other acids, such as hydrochloric, hydro- 

 sulphuric, and others, and also various salts of other subordinate metals. 



From the foregoing it appears that during the circulation of water in 

 the belt of cementation the processes of precipitation and solution are selec- 

 tive. Quartz and the hydrous silicates are the dominant precipitates. 

 Carbonates of the alkalies and alkaline earths are the dominant salts which 

 come to the surface. The above precipitations and solutions are precisely 

 what should be anticipated from the laws of chemical precipitation given 

 (pp. 113-123). The compounds which, on the average, are thrown down 

 to the g*reatest extent are those which are least soluble and most abundant. 

 The compounds which are retained in solution to the greatest extent are 

 those which are most soluble and least abundant. However, of the more 

 soluble and less abundant compounds a portion is precipitated. The 

 conditions under which we should expect partial precipitation of these 

 compounds are those of lessening temperature and pressure. These are the 

 conditions of the ascending columns of water. It has alread} 7 - been seen 

 that the ascending columns are likely to be the main water channels. 

 Hence is explained the precipitation of the comparatively soluble carbon- 

 ates of the alkaline earths and other minerals which form in the trunk 

 channels. 



By these various processes the larger openings are filled with deposits, 

 some of which contain a sufficient quantity of valuable minerals to 

 constitute ores. But where one large ojDening contains valuable minerals 

 in sufficient quantity to be of service to man, millions are filled with quartz, 

 hydrous silicates, calcite, dolomite, and other gangue minerals. It thus 

 appears that the deposition of much of the ores is but a special phase of 

 a general geological process of great consequence. This idea will be fully 

 developed in Chapter XII. 



DIFFUSION. 



Other factors besides those already considered enter into the cemen- 

 tation of openings. Of these, diffusion is important. Before considering 

 the influence of diffusion, it is necessary to recall some of the points 

 already developed. After the water enters the belt of cementation it first 

 has a downward movement, and usually later an upward movement. 



