846 



THE JOURNAL OF GEOLOGY. 



obtain where it is too thin to protect the rock beneath from 

 considerable changes of temperature, and from the effects of 

 those other disintegrating agencies which affect the surface to a 

 depth of a half dozen feet. In such situations the surface of the 



Fig. 3, illustrating the relation of drift to underlying rock. The line of contact is 

 distinct, and the rock surface is not roughened by decay. 



underlying rock may be much broken and weathered. If the 

 rock be of such a character as to successfully resist weathering, 

 its surface may be smooth and firm where the thickness of drift 

 is very slight, or even where it is altogether absent. 



The relation which the drift sustains to its underlying rock 

 bed is therefore distinctly unlike that which the residuary earths 

 of driftless regions sustain to the rock upon which they rest. In 

 the latter case, as the designation "residuary earth" implies, the 

 soil and subsoil have arisen chiefly from the decomposition and 

 disruption of the underlying rock. All rock beds lack homo- 

 geneity to such an extent that their surfaces weather unequally. 

 Their weathered surfaces are therefore uneven. The weathered 



Fig. 4 shows relation of soil to rock, where the former has arisen by the decay of 

 the latter. The line of contact is indefinite because of the irregular decay of the rock 

 surface. 



product of the rock — the subsoil — fills the little hollows on its sur- 

 face, and penetrates the fissures and cracks, while the prominences 

 of the uneven rock surface project up into the subsoil. The rela- 

 tionship referred to is illustrated in Figure 4, where the upper 

 darker portion represents the residuary earths, and the lighter part 



