WITH AGRICULTURE AND PLANTING. 471 



at length grow to such a size as to rend asunder vast 

 rocks, overturning crags and cliffs, by the power of their 

 very minute and delicate roots." The parts of rocks 

 loosened by these powers, are entirely separated, and are 

 carried to a great or less distance, by streams of water, 

 and in the higher regions, by the power of winds. In 

 cliffs and precipices which have been formed by the split- 

 ting of masses of rock, effected in the manner above 

 described, the loosened parts often lose their stability ; 

 and, following the direction of gravity, fall to the 

 ground, an effect which has also been described by Se- 

 neca in another place. " Nor is it alone probable," 

 says he, " that rocks are split asunder by their mere 

 weight, but also when streams of water are carried over 

 them, the continual moisture works into the joinings of 

 the rock, and daily takes away a portion of the connect- 

 ing matter, and, if I may so speak, abrades the skin by 

 which it is contained. At length, in the course of ages, 

 this gradual detrition so much diminishes the supporting 

 parts, that they can no longer sustain the weight. Then 

 masses of vast size fall down, and the rock tumbling 

 from its ancient seat, overwhelms whatever lies below." 

 The cohesion of some rocks, especially argillaceous ones, 

 is so slight, and their porosity so great, that their small- 

 est parts imbibe water, and are sensibly softened by it, 

 an effect which is much assisted by the freezing of the 

 water. This mechanical change is experienced by the dif- 

 ferent varieties of common clay, slate-clay, and some 

 other rocks. 



Chemical powers often act in conjunction with mecha- 

 nical ones, in breaking down rocks, the former, the che- 

 mical, frequently finishing what had been begun by the 



