22 AGRICULTURE ON THE PACIFIC SLOPE 



and dropped at this point because here the river flows more 

 slowly. On the hillsides (under the words " colluvial soil") 

 is soil which has slid down the hillside from above: it is 



UPLAND 



FIG. 12 a. The figure shows how a cut across the valley in 

 figure 12 would look. 



not as fine as that which the river brings, and does not give 

 such good crops. On the top of the hill is soil which has 

 not moved at all. It is formed just where it lies by the 

 decay of the rock underneath. 



Powdered rock and vegetation. But powdered rock 

 is not yet soil; it requires the addition of vegetable 

 , and still lurther weathering, to serve for plant 



* 



v *-^ r * 



lcanoes often throw out finely powdered rock 

 called WolcanicashJ But for a number of years this 

 ash can bear no vegetation. After several years, come, 

 1 fest, .minute plants^ called bacteria (page 250) ; then 

 v crust-like tree mosses" (or lichens), such as grow on the 

 bark of trees, begin to settle on the surface. The acids 

 given off by the lichens eat into the surface of the stony 

 particles, making them give up mineral plant food to 

 the plants. These, when they decay, give to the soil 

 some vegetable mold, or humus, which in turn enables 

 higher plants, such as the true mosses and ferns, to 



