LAND AM) WATKU. 31 



of their elevated position, they have partly escaped the 

 forces that would tend to destrcjy them. 



The process of rock-making' is still going on. There 

 is often to be found, l)eneath the surface soil, a stratum 

 of "hard pan," through which it is difficult to jicnetrate. 

 This is gradually becoming solidified by the action of 

 chemical forces, and at some time in the distant future 

 will become stone. 



The chief agencies in nature which have done the 

 work of grinding rocks, and preparing the soil of the 

 earth, are : — 



1, The Atmosphe?'e. — Nearly all I'ocks, when exposed to 

 the atmosphere above ground, undergo chemical changes 

 upon their surface by which portions are continually 

 crum])ling and falling off. This effect upon some rocks 

 will cause them to waste away in a very few years, while 

 with others the process is much slower. 



Some varieties of sandstone are found near the surface, 

 so much affected by the atmosphere, which has reached 

 them through the porous soil, as to be easily crumbled 

 into powder. 



The most enduring kinds of marble, used for erecting 

 monuments, are generally covered with fine marble dust, 

 showing that even they are not exempt from the imi- 

 versal tendency. 



2. Runnin(j Water. — With every showier and rain 

 storm, and with the melting snows in spring-time, 

 streams and brooklets are constantly wearing away the 

 rocks, and washing away soil from the sides of hills 

 and mountains, and carrying it into the rivers below. 

 As the current of the river becomes less rai)id, this soil 

 settles, forming de})Osits on the banks of the river at its 

 mouth, or on the neiglil)oring shores of the ocean. 



