Fauna and Stratigraphy of the Stormberg Series, 485 



siderations which have been fully set out by Barrell (1908) and which 

 will be briefly noticed here. 



In youthful topographic stages rock-breaking dominates over rock- 

 decay; in topographic old age the reverse is normally true. In arid 

 climates, however, and to a lesser extent in cold climates, where chemical 

 action plays a less important part than in humid and wet climates, 

 there is dominance of disintegration over decomposition even in to- 

 pographic maturity or old age. Again, in the youthful stages of to- 

 pography the waste is carried away from the higher ground as soon 

 as disintegration frees it from the rocks, in arid and subarid climates 

 by the streams arising from torrential rains, and is deposited in inland 

 basins or on piedmont slopes. Under arid and subarid conditions this 

 waste is coarser, less decomposed and has less true clay than sim- 

 ilarly situated waste of more pluvial climes. Even in maturity, the 

 mountains of arid regions retain their nakedness, roughness and 

 sterility and give rise to disintegrated waste rather than decomposed 

 material. As, however, the difference in altitude between the parent 

 mass and the sediments formed from it diminishes, so will the trans- 

 porting power of the torrents lessen, and the sediments will tend to 

 become liner in grain - the larger fragments of disintegrated mallei- 

 being unmoved by the slower and less powerful streams. 



In the old age of arid regions wind erosion becomes increasingly 

 more important than water erosion, so that the products of erosion 

 in old-age, when the desert surface is approximately Hat, are chiefly 

 wind-borne loess and dime-sand. 



A number of instances might be cited of present-day regions in 

 which one or more of these stages is seen; it is enough to quote 

 the Eastern Persian area described by Huntington, the aerial deltas 

 arising from the decaying high-lands of Arizona, and the sand-lilled 

 valleys and "island mountains" of Namaqualand in South Africa. 

 It would seem that the evidence afforded by the nature of the sedi- 

 ments, coupled with these theoretical considerations and present-day 

 examples, tends to point to the conclusion that the land-mass from 

 which the Stormberg sediments were derived, suddenly rejuvenated 

 at the end of Beaufort times, was in process of planation during 

 the deposition of the beds until the intense volcanic activity put an 

 end to sedimention in the area. 



On the other hand, there is little or no direct evidence of local 

 or regional earth-movement such as would have caused sudden reju- 

 venation at the end of Beaufort times; and the sudden onset of the 

 conglomeratic and coarse type of sedimentation which characterises 

 the Molteno Beds may have been due to more intense precipitation 



