T. S. Hunt—The Decay of Rocks. 211 
59 
ne. 
8. To these modes, with which are to be included the or- 
the one hand, and the sand and gravel of the desert steppes on 
the other. He thus explains the condition of the crystalline 
similar rocks in southern Asia, as in Brazil and the southern 
nited States, are still deeply covered with the products of 
their own decomposition. 
$49. Pumpelly further remarks that the surface of the un- 
‘decayed rock to be laid bare by erosion is necessarily an irreg- 
ular one, the inequalities depending not upon its original dif- 
ferences in hardness, but upon its resistance to decay under the 
Intiuence of atmospheric waters. The effect of fractures, joints, 
veins and dykes in the rock in favoring or retarding the action 
of this agent would be manifested by still further irregularities 
of the plane limiting the decomposition of the rock in depth. 
‘Slow downward motion of the decomposed surface on mountain-sides in North 
Carolina, as due to the alternate freezing and thawing of the contained water. 
this displaced and modified layer, which resembles that produced by glacial ac- 
tion, he gives the name of frost-drift. (Proc. Amer. Inst. M. Engineers, viii, 462.) 
