SOILS 



285 



Showing Soil Above the Shale, then 

 Solid Rock Below. 



downward the color gradually changes to a lighter brown, 

 and still deeper the stain of the decaying plants is no 

 longer seen ; the soil becomes harder and harder until we 

 finally reach the 

 solid rock. 



This is the com- 

 mon sort of soil 

 found all over the 

 world. It is made 

 by the gradual de- 

 cay of the rock 

 (Figure 246). If 

 we should remove 

 all the soil from 

 an area of rock, it 

 would immediately 

 begin to form again. After a few years' exposure to 

 the atmosphere, the rock would be decayed enough so 

 that lichens and certain mosses would find a place to 

 grow. They would aid in keeping the surface of the 

 rock moist and thus aid weathering. The decay of 

 these simple plants would produce a little humus, and 

 soon the seeds of other larger plants would germinate 

 and grow there. The decay of these plants gradually 

 adds to the layer of mold on the surface of the rock 

 until the largest trees may find a place to grow. The 

 atmosphere, the water, the decay of plants, the roots 

 of the plants, and other minor agencies are all at work 

 now deepening and enriching the soil. Of course this 

 change may take centuries, but when compared with 

 the enormous age of the earth it seems a very short 

 time. 



Glaciated Soil. In the northern portions of our conti- 



