WITH AGRICULTURE AND PLANTING. 461 



the soil upon it. In a highly inclined plane, the imperfect 

 support of the centre of gravity is the sole cause of the 

 loss of earth ; in a less inclined plane the diminution of 

 soil is usually caused by water, which produces this ef- 

 fect in a greater or less degree, according to the differ- 

 ence of inclination. In both these modes, by which a re- 

 moval of soil is produced, the effect may be modified by 

 a difference in the condition of the loose earth, as not 

 only its stability as to situation, but also its resistance 

 to the power of water, vary according to the size, fi- 

 gure, and cohesion of the parts, as well as their adhesion 

 to the surface of the rock. Sandy loose soils, for ex- 

 ample, are more liable to transposition than marly or 

 loamy ones; and these, again, are more easily moved 

 than such as are clayey and adhesive. 



Whatever be the nature of the soil, a small degree of 

 inclination in the solid rock is sufficient to favour its de- 

 nudation by the removal of the former ; and the inclina- 

 tions of the surfaces of rocks having a covering of earth 

 and vegetation, are in reality much less considerable 

 than we usually suppose them to be, judging merely by 

 the eye. The celebrated Humboldt has published ob- 

 servations on this subject. According to his measure- 

 ments, a slop of even fifteen degrees appears steep, and 

 a declivity of thirty-seven degrees is so abrupt, that if it 

 be covered with a dense sward, it can scarcely be climbed. 

 The inclination of the pastures of the Alps seldom ex- 

 ceeds an angle of ten or fifteen degrees, and a slope 

 of twenty degrees is pretty steep. At an inclination of 

 forty degrees, the surface of the rock is sometimes covered 

 with earth bearing a sward, but at a greater inclination 

 the I'ocks are usually destitute of soil and vegetation. 



