CHEMICAL DEPOSITS IN SPRINGS. 77 



stalactites (Fig. 72), a a, and stalagmites, b b, which, meeting each 

 other, form limestone pillars, c c. The great Mammoth Cave, in Ken- 

 tucky ; Wier's Cave, in Virginia, and K" icojack Cave, in Tennessee, are 

 familiar examples. As might be expected, subterranean rivers are 

 often found in these caves. This is the case in the Mammoth Cave 

 and in Xicojack Cave. 



Thus, as the same river will erode or deposit according as it is 

 under-loaded or overloaded with sediment ; so the same underground 

 stream may hollow out passages by solution or fill them up by deposit 

 according as its waters are under-saturated or over-saturated with min- 

 eral matter. 



There are many other effects of subterranean waters of the greatest 

 importance, such as the formation of fossils, the filling of mineral 

 veins, the metamorphism of rocks, etc. ; but these will be taken up 

 each in its appropriate place. 



Chemical Deposits in Springs. 



Deposits of Carbonate of Lime. — We have just seen that ordinary 

 subterranean waters in limestone districts, and, therefore, containing 

 small quantities of carbonate of lime, deposit this substance only very 

 slowly by drying, . as stalactites and stalagmites; but in carbonated 

 springs in limestone districts a very rapid deposit of lime carbonate 

 often occurs. 



Explanation. — In order to understand this, it is necessary to re- 

 member : 1. That lime carbonate is insoluble in pure water, but soluble 

 in water containing carbonic acid ; 2. That the amount of carbonate 

 dissolved is, up to a limit, proportionate to the amount of carbonic acid 

 contained ; 3. That the amount of carbonic acid which may be taken 

 in solution by water is proportionate to the pressure. 



Now, there are two sources of carbonic acid, viz., atmospheric and 

 subterranean. All water contains carbonic acid from the atmosphere, 

 and will, therefore, dissolve limestone, but this deposits only slowly by 

 drying, as already explained. But in many districts, especially in vol- 

 canic districts, there are abundant subterranean sources of carbonic 

 acid. If subterranean waters come in contact with such carbonic acid, 

 being under heavy pressure, they will take up a large quantity of this 

 gas ; and if such water comes to the surface, the pressure being re- 

 moved, the gas will escape in bubbles. This is a carbonated spring. 

 If, further, the subterranean waters, thus highly charged with carbonic 

 acid, come in contact with limestone rocks, or rocks of any kind con- 

 taining lime carbonate, they will dissolve a proportionately large amount 

 of this carbonate ; and when they come to the surface, the escape of 

 the carbonic acid causes the lime carbonate to deposit abundantly. 

 Thus around carbonated springs in limestone districts, and along the 



