202 GEOLOGY 



taken up by plants and is passed by them into the atmosphere ; some 

 of it issues in the form of springs; some of it seeps out; some of 

 it is drawn out through wells; and much of the remainder finds its 

 way underground to the sea or to lakes, issuing as springs be- 

 neath them. A small portion of the descending water enters into 

 permanent combination with 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 thereby decreasing. Minerals 

 once hydrated may be dehydrated, the water being set free. Fur- 

 thermore, considerable quantities 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. The amount of water reach- 

 ing the surface of the earth for the first time from volcanoes, may, 

 so far as now known, equal or even exceed the amount consumed 

 in the hydration of minerals. 



THE WORK OF GROUND-WATER 



Ground-water is ever active. The results of its work are partly 

 chemical and partly mechanical, the former being the more im- 

 portant. 



The 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 solution of mineral matter. Pure 

 water will dissolve certain minerals; but the carbon dioxide ex- 

 tracted from the atmosphere, and the products of organic decay 

 extracted from the soil give ground-water a power to dissolve which 

 is not possessed by pure water. The solvent work of ground-water 

 is shown by the fact that all such water, whether it issues as springs 

 or is drawn out through wells, contains mineral matter, while rain 

 water is essentially free from it. The subtraction of soluble matter 

 from rock tends to make it porous, and helps it to decay. 



2. One mineral substance in solution may be exchanged for 

 another extracted from the rock. Thus the lime carbonate of a 



