90 RUNNING WATER 



considerable size flowing in them. Indeed, in iimestone regions 

 the smaller streams are generally engulfed and flow for a longer 

 or shorter distance underground. When the roof of a cavern is 

 no longer able to support itself, it falls in and exposes a ravine. 

 Any portion of the roof which remains standing will then form a 

 bridge, examples of which are the famous Natural Bridges of 

 Virginia and of the Tonto Basin in Arizona. 



Nothing is known as to the limits of depth to which the perco- 

 lating waters may penetrate the crust of the earth, but so far as 

 borings and deep mines have gone, water is always found, and the 

 limit is probably far below any yet reached artificially. As the 

 temperature of the earth increases downward, a level must be 

 attained (probably at no very great depth) where the rocks 

 become too hot to allow any further penetration, and at such 

 depths the great pressure of the overlying masses must tend to 

 close up the joints and crevices through which the water descends. 

 The moisture in deep-seated rocks must be, for the most part, 

 stationary or subject only to very slow fluctuations, for such rocks 

 are solid and undecomposed. Even beds of rock salt, which 

 would surely be dissolved away by moving water, are found at 

 depths which can be reached by mining or boring. 



When underground waters become highly heated by descending 

 to great depths along channels which admit of a return to higher 

 levels, or by coming in contact with masses of hot lava, their 

 solvent efficiency is greatly increased. Rocks penetrated by such 

 thermal waters are often profoundly altered in their character 

 and composition. In igneous rocks so treated the complex 

 minerals which make up these rocks are decomposed into simpler 

 or more stable compounds. The felspars become opaque from 

 the formation of kaolin, or are transformed into hydrated mica ; 

 minerals containing iron and magnesia give rise to chlorites 

 (p. 21), serpentine (p. 22), and the like, while the lime com- 

 pounds are converted into the carbonate and carried away in 

 solution. Some of the minerals are altered in place, and others 

 are deposited in the fissures and cavities of the rock. Thermal 

 waters also alter the character of rocks by bringing in new 



