222 GEOLOGY, 



mineral matter. Many minerals are known to take up water, being 

 changed thereby from an anhydrous to a hydrous condition. It does 

 not necessarily follow, however, that the total supply of water is there- 

 by decreasing. Minerals once hydrated may be dehydrated subse- 

 quently, the water being set free. Furthermore, considerable quan- 

 tities of water in the form of vapor issue from volcanoes, and volcanic 

 vents often continue to steam long after volcanic action proper has 

 ceased. It is probable that some, and perhaps much of the water 

 issuing from these vents has never been at the surface before, and it 

 is not now possible to afhrm that the supply from this source does not 

 offset, or even surpass, the depletion of the hydrosphere resulting from 

 mineral hydration. 



THE WORK OF GROUND-WATER. 



Ground-water effects very considerable results in the course of its 

 history. These results are partly chemical and partly mechanical, 

 the former being far more important than the latter. 



Chemical Work. 



The results of the chemical and chemico-physical action of water 

 may be grouped in several more or less distinct categories. 



1. The simplest effect is the subtraction of soluble mineral matter. 

 Pure water is in itself a solvent of certain minerals; but the carbonic- 

 acid gas extracted from the atmiosphere, and the products of organic 

 decay extracted from the soil make ground-water a much more efficient 

 solvent. Something of the results which it achieves is shown by its 

 composition. All ground-water, whether issuing as springs or drawn 

 out through wells, contains much more mineral matter than the water 

 which falls as rain, and the excess is acquired in its underground course. 



The subtraction of soluble matter from rock renders it porous. The 

 amount of material dissolved from a given place may be trivial or con- 

 siderable, according to the character of the rock, the readiness with 

 which water has access to it, and the character of the water. Locally, 

 the subtraction of mineral matter may be the chief, or even the only 

 appreciable, effect of the ground-water. 



2. It sometimes happens that ground-water with certain mineral 

 substances in solution exchano;es them for other substances extracted 



