THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 395 



granite being the most frequent; the plains are of more easily eroded 

 rocks, such as gneiss, schists, slates, sandstones, and limestones. 

 The bedding of the rocks is not flat, but disturbed; the plain therefore 

 truncates the rock structures. The rocks are not deeply decom- 

 posed, but are relatively fresh. The products of weathering are 

 usually spread as a thin veneer on the plain; the waste does not lie 

 in place, on the rocks from which it was weathered, but has been 

 drifted about by wind and flood, and has gathered in slight depres- 

 sions. The waste veneer increases the smoothness of the plain, but 

 the rock surface is also a plain, as may be seen in the edge of water 

 channels, as well as where the veneer is absent (b, p. 195). Neigh- 

 boring areas contain extensive deposits of irregular strata, whose 

 composition and want of fossils indicate their desert origin, as will 

 be referred to again below. Various additional details are given, 

 with the conclusion as above quoted: these rock-floored plains are 

 not uplifted peneplains, but are the product of desert erosion unre- 

 lated to normal baselevel, in which occasional water-action has 

 co-operated with more persistent wind-action. 



The scheme of the arid cycle thus seems to be as well supported 

 by appropriate facts as is the scheme of the normal cycle; it is, 

 indeed, in one respect even better supported, for while the arid 

 African plains are examples of old desert plains now growing still 

 older, it is difficult to point out any large peneplain that still stands 

 close to the baselevel with respect to which it was worn down. 



Contrasted consequences 0} normal baseleveling and desert-leveling. 

 — While the theory of marine planation was in vogue, it was custom- 

 ary to interpret all evenly truncated uplands — that is, uplands whose 

 surface truncates their rock-structure — as uplifted plains of marine 

 abrasion, more or less dissected since they were uplifted. When 

 the efficacy of subaerial erosion was recognized, it became equally 

 customary to interpret truncated uplands as once baseleveled and 

 afterward uplifted peneplains. If Passarge's views be now accepted, 

 it follows that no truncated uplands should, without further inquiry, 

 be treated as having been eroded when their region had a lower 

 stand with respect to baselevel; the possibility of their having been 

 formed during an earlier arid climate as desert plains, without regard 

 to the general baselevel of the ocean, must be considered and excluded 

 before baseleveling and uplift can be taken as proved. 



