Earth Study 



843 



But there are other agencies besides 

 water that help in grinding the stone flour. 

 If we visit some rocky cliff, we are sure to 

 find at its base a heap of stones, gravel and 

 soil, which the geologists call talus. In our 

 eastern country we know that these pebbles 

 and soil were pried loose by Jack Frost 

 with his ice wedges. The water filters into 

 all the cracks and crevices of the rock, and 

 since water, when freezing, is obliged to 

 expand, the particles of rock were thereby 

 torn loose and forced off and fell to the 

 bottom of the cliff. Moreover, rocks 

 expand when hot, and are often thus 

 broken without the aid of water and frost. 

 In the rocks of the desert, the changes in 

 temperature pry off the rock particles, 

 which the winds carry away to make up 

 the sands of the desert. The winds hurl 

 these sands against other rocks which are 

 still standing, and hurl them with such force 

 that more particles are torn off, making 

 more sand. In fact, the wind, in some 

 regions, grinds the rocks into stone flour 

 as effectually as does the water in other 

 places. Then, too, the gases of the air also cause rocks to decay. We 

 know how iron rusts and falls to pieces through contact with the gases 

 of the air. Some rocks decompose in a similar way. We often see that 

 the inscriptions on old headstones have been almost obliterated, because 

 the gases in the air have so decomposed the marble. 



In addition to the other soil makers, there are the little plants which we 

 call lichens. The spores of these plants are so minute that we cannot see 

 them, and they drift about in the air until they find resting place upon some 

 rock. Here they begin to grow, and as they grow they become strongly 

 acid ; they are thus enabled to eat a foothold into the rock, softening its 



One of Uncle John's nieces making 

 stone flour. 



Lichens growing on rocks. 

 Photo by Verne Morton, 



