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of certain constituents. Thus, the action of water charged with carbonic acid 

 and oxygen gives rise, in many cases, to the decomposition of the original 

 silicates, and the removal of certain constituents. The action is best 

 illustrated in the basic group of rocks. The first stage appears to consist 

 in the formation of carbonates, and the liberation of a certain amount of silica 

 which may or may not be removed in solution. A basic rock in this stage of 

 alteration will frequently effervesce under the action of an acid almost as 

 readily as a limestone. The next stage consists in the removal of the 

 protoxide bases lime, magnesia and iron in the form of soluble bicarbonates. 

 If this solution be subjected at any point to oxidising influences the iron is 

 precipitated as hydrous ferric oxide ; but if not it may be totally removed from 

 the rock along with the other constituents, and thus give rise to the chalybeate 

 springs. The final result of this alteration is the formation of a loose powdery 

 substance mainly composed of hydrated silicate of alumina, in which however, 

 a large part of the original soda is often retained. The process above 

 described may of course be considerably varied in certain circumstances. 

 Thus, if pyrites be present the oxidation of this substance will give rise to 

 sulphuric acid, which is a powerful solvent, and may even remove a portion 

 of the original alumina as a sulphate. Under other circumstances the lime 

 may be wholly and the iron partially removed, while the magnesia remains 

 behind as a hydrous silicate in the form of serpentine. The general effect 

 of the simple alteration of igneous rocks by surface agencies may be described 

 as the removal of some of the bases, and a certain amount of the silica 

 originally combined with these bases, and the addition of water and oxygen. 

 Some details as to the nature of the chemical alteration to which rocks are 

 subjected will be given when the individual rocks are described. 



