28 AGRICULTURE. 



been from very early geological times, and still 

 are, a powerful, though silent, factor in the pro- 

 cesses of rock disintegration and soil formation 

 is too often overlooked or underestimated. 



(i) MECHANICAL OR PHYSICAL EFFECTS. 

 Generally, wherever rock has been acted upon 

 by the processes of weathering, vegetation 

 creeps in. It may be some .very low form, as 

 fungus, moss, or lichen, but it sends its tiny 

 root-like extensions into the crevices of the rock 

 and forces apart its particles. 



In the higher forms of vegetation, where the 

 roots are strong and woody, this becomes an 

 important feature (Fig. 9). Huge boulders are 

 burst asunder by the root-pressure of some giant 

 tree; through innumerable rocky crevices larger 

 or smaller root systems are finding their way, 

 opening up the solid rock, and rendering it sus- 

 ceptible to other disintegrating forces (Fig. 9). 

 In this same way myriads of grass roots and 

 roots of herbs and forest trees are pulverizing 

 the solid material of the soil. 



While plants absorb water from the soil, at 

 the same time, where vegetation is at all dense, 

 they shield the earth's surface from the direct 

 rays of the sun so effectively as to retard evapo- 

 ration. This retained moisture exerts a solvent 

 power upon the rock substances. 



Grasses, or other plants having thick, matted 

 roots, prove a great protection from the median- 



