ACTION OF THE GASES. 63 



be very small. It would therefore have a greater penetrating power than 

 water, and would be more highly energetic in its action. Under these 

 conditions the minutest spaces would be somewhat readily traversed. The 

 rocks of the deep zone in which action of this kind has taken place can 

 reach the surface only by passing through the zone in which water is in the 

 liquid form. Therefore the effects which were produced by the mineralizers 

 in the deepest zone will have been modified by the action of water solu- 

 tions during- the long time the rocks were in the belt of cementation. The 

 details of the effect of water gases in the zone of rock flowage will be con- 

 sidered in Chapter VIII, on "The zone of anamorphism." 



All of the gases may act in either of two ways: (1) By their presence 

 they may influence crystallization or recrystallization without entering into 

 combination. (2) They may enter into the combinations forming oxides, 

 hydroxides, carbonates, sulphates, etc. The first of these actions is spoken of 

 as that of crystallizers, and the second as that of mineralizers. In the meta- 

 morphic rocks it is ordinarily difficult to prove the past action of gases, not 

 in water solutions. Occasionally the materials of volcanic cones have been 

 rendered porous and the rocks altered in consequence of the action of 

 gaseous exhalations. In such cases the gases usually have united to some 

 extent with the materials through which they have passed, and in this way 

 furnish evidence of their past action. 



PART II. AQUEOUS SOLUTIONS A3STD SOLIDS. 



GENERAL CONSIDERATIONS. 



The one liquid through which the greater part of the alterations of 

 rocks occur is water solution. Indeed, this is so profoundly true that the 

 water of the earth has been compared with the blood of an organism. And it 

 is certainly true that the transformations of tissues by the blood are scarcely 

 more far-reaching than those of the lithosphere by the agency of water. It 

 has been determined by laboratory experiments that pure water at ordinary 

 temperatures is capable of dissolving all compounds to some extent. Cor- 

 responding with this fact, analyses of ground waters show that they contain 

 in solution all of the elements which occur in nature. The solutions may 

 vary from very dilute to rather strong. So far as the gases are dissolved in 

 water, their action is to be treated under water solutions, not under gases. 



