ATMOSPHERIC AGENCIES. 



11 



the soil still rests on the rock from which it was made. 

 Unfortunately this is rare in the northern part of our 

 country, where the soil has been nearly everywhere shifted 

 during a period which we shall hereafter describe as the 

 Drift period. But in the southern part of the United 

 States, on all the hillsides and mountain-tops, the soil 

 has been undisturbed for ages, and the evidence is com- 

 plete, and may be observed by any one. If, for example, 

 we note carefully the sections made by railroad and well 

 diggings, we shall see at the top perfect soil, perhaps red ; 

 a little deeper it becomes lighter colored and coarser 

 grained ; then it begins to look like rotten rock ; and, 

 finally, by insensible degrees, it passes into sound rock. 

 The evidence is still more complete if, as is often the 

 case, the rock is traversed by a quartz-vein. In such a 

 case we can trace the quartz-vein through the sound 

 rock, and upward through the rotten rock, the imperfect 

 soil, and the perfect soil, to the surface, where it may 

 usually be traced over hill and dale as white fragments 

 lying on the surface. The reason is this : Quartz is a 

 mineral which will not disintegrate under atmospheric 

 agency ; therefore it remains sound, while all the rest of 

 the rock is changed into soil (Fig. 1). 



FIG. 1. Section and perspective view (ideal), a, sound rock ; &, rotten rock ; 

 c, perfect soil ; d, quartz-vein ; c?', same, outcropping on surface ; e, mass of 

 more resistant rock imbedded in soil. 



Sometimes a rounded mass of sound rock, e, is seen 

 imbedded in the soil. This is only a harder piece of 



