400 W. M. DAVIS 



planation, as shown by Lapparent (b). Disturbances of the arid 

 cycle are followed by consequences of other kinds. 



Interruptions and modifications oj the arid cycle. — A land mass 

 suffering erosion under an arid climate may, as in the normal cycle, 

 suffer interruption in the regular progress of its changes by movement 

 of any kind at any stage of development. If, for example, integration 

 of drainage has advanced so far that the number of original basins 

 is reduced by half, the number may be increased again by renewed 

 deformation; or if the integration of drainage has reached a mature 

 stage, the drainage may be thrown into disorder again by a more 

 or less gentle warping of the region. In all such cases a new cycle 

 may be regarded as having been initiated; its initial forms will 

 be the eroded forms developed during the preceding incomplete 

 cycle, and displaced by the movements through which the preceding 

 cycle was closed. The work of the new cycle, thus initiated, then 

 goes on as before; but with interruptions of this kind we are not here 

 particularly concerned, because they offer no special difficulty of 

 explanation or interpretation. 



It is otherwise when interruptions or modifications of the arid 

 cycle occur after old age is well advanced, for -the desert plains then 

 developed may, under certain conditions, come to imitate uplifted 

 undissected peneplains, as has already been partly considered in 

 the preceding section. 



Uniform uplift or depression, by which a normal peneplain is so 

 immediately and significantly modified, will not interrupt the regular 

 process of degradation on a desert plain in an arid cycle. . It is 

 perhaps in part for this reason that actual examples of rock-floored 

 desert plains appear to be more common than actual examples of 

 peneplains. Depression would drown a peneplain, and elevation 

 would cause its dissection; but, unless carried to an extreme, neither 

 of these movements would greatly affect the slow degradation of a 

 desert plain. Unequal movements, whereby a desert plain is warped 

 or slanted, are of more importance and are probably of more common 

 occurrence. 



If an old rock-floored desert plain be gently warped or tilted, 

 marine submergence is not likely to follow immediately, but the 

 regular continuation of general degradation will be interrupted. The 



