CHAPTER 11. 



SPRINGS. 



Mark what happens when a heav>- shower of rain falls upon 

 dry ground. If the ground be formed of hard and soHd 

 rock, such as granite, the rain, after wetting the surface, runs 

 off in all directions ; some finding its way to the nearest 

 streamlet, whence it flows sooner or later into a river, and 

 some finding lodgment in little hollows of the rock, where 

 it collects in pools which are slowly dried up by wind and 

 sunshine. But if the ground, instead of being hard like 

 granite, is soft and porous like sand or chalk, the water 

 will then sink into its substance, and may even pass out of 

 sight without so much as wetting the surface of the thirsty 

 soil. Rocks which thus allow water to filter through them 

 are said to be permeable, while those which refuse to allow 

 the water to soak in are said to be impermeable: a bed of 

 sand, for example, is permeable; a bed of clay im- 

 permeable. 



It is by no means necesssary, however, that a rock, in 

 order to be porous and permeable, should be either soft 

 like chalk, or loose like sand. Take for instance a sand- 

 stone, or a hard limestone : these rocks are sufficiently 

 coherent to form durable building stones, yet porous enough. 



