76 



AQUEOUS AGENCIES. 



ficial and subterranean, into the sea. Their relative amount it is impos- 

 sible to determine. Much depends upon the configuration of the coun- 

 try and the nature of the strata. The heavy hydrostatic pressure to 

 which subterranean water is subjected, especially in elevated countries, 

 brings the larger portion of it to the surface as springs. But, in lime- 

 stone regions (this rock being affected with frequent and large fissures, 

 and open subterranean passages, as will be hereafter explained), large 

 subterranean rivers often exist, and these, even after coming to the sur- 

 face are often re-engulfed, and finally reach the sea by subterranean 

 passages. The same is true also of regions covered with recent lava- 

 flows; for these also are full of caves and galleries (page 92). ' The 

 largest springs, therefore, generally occur either in limestone or in vol- 

 canic countries. From the Silver Spring, in Florida, issues a stream 

 navigable for small steamers up to the very spring itself. The country 

 for sixty miles around is entirely destitute of superficial streams, the 

 whole drainage being subterranean, and coming up in this spring.* 

 About Mount Shasta all the streams head in great springs. 



Chemical Effects of Subterranean Waters. — We have already seen 

 (page 6) how atmospheric water disintegrates rocks, dissolving out 

 their soluble parts, and reducing their unsoluble parts to soils. Springs, 

 therefore, always contain these soluble matters. In granite regions 

 they contain potash ; in limestone regions they contain lime, and are 

 called hard ; in other cases they contain salt, and are brackish ; when 

 the saline ingredients are unusual in quantity or in kind, they are 

 called mineral waters. 



Limestone Caves. — In most rocks, the insoluble part left as soil is 

 far the largest, only a small percentage being dissolved. In such rocks, 



therefore, the resulting 

 soil fills the whole space 

 originally occupied by 

 the rock. But in the 

 case of limestone the 

 whole rock is soluble. 

 Therefore, in limestone 

 regions, percolating wa- 

 ters dissolve the lime- 

 stone, hollow out open 

 passages, and form im- 

 mense caves. Water 

 charged with limestone, dripping from the roofs and falling on the 

 floors of these caves, deposit their limestone by evaporation, and form 



Fig. 72.— Limestone Cave. 



* The exceptional transparency of limestone waters is due to the property, possessed 

 by lime in a remarkable degree, of flocculating and precipitating clay sediments. 



