12 DYNAMICAL GEOLOGY. 



rock, which has resisted disintegration, while the rest has 

 yielded. These are called bowlders of disintegration. 



It is not always, even in lower latitudes, that we find 

 this gradation between soil and rock. Often perfect soil 

 is found to rest on sound rock, with sharp limit between. 

 In all such cases there has been shifting of the soil. In 

 northern latitudes (37-40 northward), as already stated, 

 the soil nearly everywhere rests on sound rock, and often 

 the underlying rock is smooth and polished. We shall 

 explain this hereafter. But even in the Northern States, 

 if one will notice closely, he will see the process of soil- 

 making going on. Rock-fragments, which were once an- 

 gular, become rounded by rotting of the corners. Cliffs, 

 by their crumbling, gather piles of rock-fragments and 

 earth (talus) at their bases (Fig. 2). The pupil ought to 



FIG. 2. Cliff, showing talus, , and bowlders of disintegration, 6, b. 



observe these things habitually, as it is on just such 

 observation of simple things that true science rests. 



Depth of Soil. Since soil is constantly carried away 

 by washing of rain, as will be more fully explained in the 

 next chapter, it- is evident that there are two opposite 

 processes here to be considered, viz., soil-formation and 

 soil-removal. The depth of the soil will depend on the 

 relation of these two to each other. More definitely, the 

 depth of the soil depends partly upon the kind of 'rock 

 (for this affects the rate of formation), and partly on the 

 slope (for this affects the rate of removal). On high 



