ATMOSPHERIC AGENCIES. 



13 



slopes the rock is bare (Fig. 3,, a), not because there is no 

 soil formed, but because it is removed as fast as formed. 





FIG. 3. , sound rock ; 6, rotten rock ; c, soil formed in place ; d, soil 

 shifted from e. 



On flat lands, near high slopes, the soil is deep (Fig. 3, b), 

 because not only is it formed here in place, but the wash- 

 ings from above are added. 



Bate of Disintegration. If rocks were solid, so that 

 the agents of decomposition could act only on the sur- 

 face, the rate would be inconceivably slow, but all rocks 

 are affected with joints in several directions, by which 

 the mass is divided into more or less separable blocks, so 

 that a cliff looks something like a wall of regularly piled 

 blocks without cement (Fig. 2). Water, therefore, pene- 

 trates to great depths, attacking the surface of every 

 block. Also, every block is itself affected throughout 

 with capillary fissures, through which water penetrates to 

 every part (quarry-water of stone-cutters). Thus, the 

 rocky crust of the earth is affected by disintegrating 

 agencies to very great depths though, of course, most 

 rapidly at the surface. 



Bowlders of Disintegration. All over the Northern 

 States are found scattered rock-masses (bowlders), lying 

 on the surface. If we examine these, we shall usually 

 find that they are entirely different from the country- 

 rock. They have been brought from a distance how, 

 we shall explain hereafter. We have nothing to do with 



