LESSONS IN GEOLOGY. 341 



The effect on the rocks of this underground water is much the 

 same as the effect of air and water on the surface rocks. Solu- 

 tion and disintegration are usually more rapid in limestone rocks, 

 large channels and caves being common in such formations. 

 Sometimes the roof of the cave falls in and the cave or under- 

 ground channel becomes a gorge or narrow valley. The caves of 

 Southern Indiana, Kentucky and Tennessee are remarkable for 

 their size and the beauty of their stalactitic architecture. In 

 these caves there are often interesting examples of reparative 

 action. In cases where the bed of the stream has been lowered so 

 as to leave some rooms practically dry, the dripping from thereof 

 at length forms pillars, supporting the roof, and in time perhaps 

 fills the room. Again, where quantities of rock have fallen, the 

 drippings from above fill the spaces between the fallen rocks with 

 stalagmitic material and at length build supporting columns, 

 finally closing the cavity. 



Sometimes the underground water, on sloping surfaces, dis- 

 solves away the union between large masses of rocks and the 

 mountain side, causing land slips, rock avalanches, etc. 



