458 A TREATISE ON METAMORPHISM. 



in rocks. In the ground water they are ordinarily composed of com- 

 pounds consisting of the bases sodium, potassium, calcium, magnesium, 

 iron, and aluminum, and the acids, carbonic, hydrochloric, nitric, hydro- 

 sulphuric, sulphuric, and phosphoric, and also colloidal silicic acid. Nitrous, 

 sulphurous, and organic acids are also present. These bases and acids unite 

 in various ways to produce many salts, the majority of which are, however, 

 simple salts, such as the carbonates of the alkalies and alkaline earths. 



Of these salts the dominant ones are those of sodium, potassium, 

 calcium, and magnesium in the form of carbonates, sulphates, and chlo- 

 rides. Furthermore, there are present other bases and acids which are 

 omitted because of their subordinate quantity. For instance, in some 

 cases various salts of manganese, copper, silver, g*old, etc., are present, 

 but the quantities of such compounds are so minute as to have no sig- 

 nificance except in connection with ore deposits. 



The water, moreover, contains oxygen, carbon dioxide, and other 

 gases in solution. Where the rocks are not saturated by water gaseous 

 solutions also are present and active. Of these oxygen and carbonic acid 

 are the most important active chemical agents. 



While it is well established, as explained (p. 63), that pure Avater acts 

 upon all of the compounds which occur in nature, the activity of solutions 

 is very greatly increased by the mineral content. This is true of bases, of 

 acids, and of salts alike. Where bases or acids are present it is well known 

 that the activity of solutions is greatly increased; but it has not been always 

 fully appreciated that the same is true in reference to normal salts. 



Therefore all of the above compounds, in all their forms, are actively 

 at work in decomposing the rocks. The amount of work which any one 

 compound does depends upon the strength of its bases or acids, and also 

 upon its quantity, or upon the law of mass action. While if the compounds 

 are present in equal quantity the stronger bases or acids do more work, it 

 is frequently the case that a weaker compound more than compensates for 

 this weakness by its great abundance. Illustrating this is carbonic acid, 

 which, while very weak, is on account of its abundance, one of the most 

 potent agents in the alterations of rocks in the belt of weathering. 



(b) The temperature is of very great importance in the activity of 

 the water and g-aseous solutions. 



