AQUEOUS AGENCIES. 71 



underground drainage. The relative proportion of these 

 varies greatly in different countries, depending upon the 

 nature and position of the rocks ; but by. far the larger 

 part usually returns by the surface, being brought up by 

 hydrostatic pressure. Only in limestone and in recent 

 volcankT regions is the proportion returning by under- 

 ground passages large. 



Chemical Effects of Underground Waters ; Min- 

 eral Springs. We have seen that all rocks are changed 

 into soils by the removal of their soluble portions. These 

 are then taken up by percolating water and brought to the 

 surface by springs, while the insoluble portions remain as 

 soils. It is evident, then, that all springs contain mineral 

 matters derived from the rocks. If the quantity be large, 

 or the mineral rare and medicinal, then it is called a min- 

 eral spring. These are usually hot because of the great 

 solvent property of hot water. 



Limestone Caves. Now, in most cases the propor- 

 tion of insoluble matter is so large that the resulting soil 

 is fully as bulky as the rock from which it was formed ; 

 there is, therefore, no vacant space left. But, in the case 

 of limestone, the whole rock is soluble in water containing 

 C0 2 . Underground streams, therefore, dissolve out gal- 

 leries and caves in their courses. Hence, irregular caves 

 and galleries are found in limestone rocks in all countries. 

 These are often of very great extent, the galleries being 

 sometimes, as in the case of the Mammoth Cave, hun- 

 dreds of miles long. The most celebrated in this country 

 are the Mammoth Cave, Kentucky ; Wyandotte Cave, 

 Indiana ; AVier and Luray Caves, Virginia ; Nicojack 

 Cave, Tennessee ; and Bower Cave, in California. 



In all cases they have been hollowed out by solution and 

 by erosion, and therefore are, or have been, occupied by 

 underground streams. Some are so still, as the Nicojack ; 

 some only partly, as the Mammoth Cave ; some not at all. 

 In all cases, they have been occupied in former times by 



