HOW TO OBSERVE. 



65 



Again : a person who first begins to observe geo- 

 logical phenomena is very apt to mistake an under 

 for an upper stratum. For example, suppose (in 

 figure 20) a hill to be covered with vegetable soil, and 



Fig. 20. 



a quarry or pit to be made in it near the bottom, as 

 at a, and the stone was discovered to be sandstone ; 

 if another pit was sunk higher up at b, which cut 

 into limestone, it might be supposed, because the 

 limestone is met with at a higher level, that it lies 

 over the sandstone stratum, when it is, in reality, 

 below it. And so at c, at the top of the hill, he 

 might suppose that the slate which crops out on the 

 summit was above both the others, when, in fact, it 

 is the lowest of the three in the order of position, 

 though the highest in point of elevation. 



We often read in geological works of valleys of 

 denudation ; these may be explained by the follow- 

 ing figure. 



Fig. 21. 



Here we perceive that several tracts of the upper 

 strata are wanting between a and b (fig. 21), and that 

 the lower strata have been laid bare either by the 

 F 



