436 ME. A. J. C. MOLYNEUX ON THE [Aug. I909, 



which it is facilitated are described in the following section of this 

 paper (§ VI). The area of these regions, and of the Luangwa 

 Valley, where probably the same general conditions exist, is some 

 30,000 square miles ; and, when it is mentioned that from 1000 

 to 2000 feet of superimposed strata have been removed since 

 the last elevation of the plateau, this conveys a mere hint of the 

 potency of the erosive forces that have been and are still at work. 



VI. Stjbaekiai Decomposition ibt the Ltjano. 



In the degradation of the Karroo deposits of the Luano Valley, 

 rivers and storm-courses have cut down the overlying beds to a 

 fairly even plain ; indeed, it is remarkable that there are no deeply 

 incised gorges or high hills (except the Archaean inliers), as one 

 might expect in areas that are subject to much erosion. 



This is explained by the complete absence of strata that possess 

 any powers of resistance, such as the effusive basaltic sheets that cap 

 other areas, or the massive sandstones of the Sengwe and Wankie 

 districts. There are some siliceous and calcareous deposits that 

 are tough when freshly excavated ; but, generally, atmospheric 

 decay brings about rapid disintegration, and any rocks that crop 

 out are soft and friable. 



Towards this removal of the beds much is due to decay of the 

 rocks in situ. Where there are flat areas of unstratified clays or 

 argillaceous sandstones, the surface, generally covered by mapani 

 trees, soddened in summer and turned to a clinging mud by the 

 heavy rains that hardly find a drainage-slope, dries up during the 

 general desiccation in the seven dry months of winter and forms a 

 maze of cracks, nearly as wide as a man's boot, and thinning down- 

 wards for 50 feet. In addition, deeper clays dry up, and pulverize 

 into fragments. Down the cracks the air, moisture, and acids, 

 derived from the rotting of surface-vegetation, penetrate, and by 

 these means the original structure of the rock is lost, calcareous 

 or ferruginous strata are converted into a loose sand, and the clays 

 into a friable mass. Grass and tree-roots aid in the general 

 reduction. Carbonaceous beds seem to suffer most, for the per- 

 colating agents act on the pyrites ; and, heat resulting naturally 

 from the chemical change, expansion takes place, which twists the 

 stratum or brecciates it, and even brings about slickensiding move- 

 ments in the superincumbent deposits. "When decomposition is 

 complete and the soluble contents are removed, the bed shrinks, 

 and subsidence takes place. 



In this way, areas of soft material are formed, and it will be 

 realized that they fall a ready prey to fluviatile erosion. At times 

 these sink into saucer-like depressions, to be puddled even deeper 

 by wallowing animals when storm-water collects. Frequently 

 also, the desiccation-cracks develop into drains and undermine the 

 surface in a chain of gaping holes, eventually to declare themselves 



