SOLUTION BY UNDERGROUND WATERS 



8 9 



assumption that because the former was farther down the hill, it 

 could not contaminate the latter. But the inclination of the rocks 

 is such that the cess-pool would drain as directly into the well as 

 though a pipe connected them. Underground waters perform the 

 work of rock disintegration, both chemically and mechanically, 



FIG. 28. — Diagram illustrating how surface and underground drainage may be in 

 opposite directions. 



but as the movement of such waters is usually extremely slow, the 

 mechanical work is of very subordinate importance. 



In considering the effects of the rain we learned that its chem- 

 ical efficiency is much increased by the humous acids which it 

 takes up on passing through the soil. The water, making its way 

 downward through the rocks, by means of the joints and bedding 

 planes, exerts its slowly dissolving and decomposing effects upon 

 the walls of these crevices. Such water therefore always contains 

 more or less mineral matter in solution, the nature and quantity of 

 which depend upon the character of the rock traversed. 



In passing through limestones, percolating waters produce re- 

 markable effects, owing to the solubility of the rock. From the 

 surface sinkholes and pipes are dissolved downward, while in the 

 mass of the rock caverns are dissolved out, often, as in the Mam- 

 moth Cave of Kentucky, many miles in extent and with rivers of 



