424 Notices of Memoirs — Denudation of Arid Regions. 



Though, the main features of a desert land depend on the geological 

 structure and in part on past climatic conditions, there are 

 characteristics which are typical of all arid regions. These are far 

 removed from the great marine areas and from the zone of rainfall 

 dependent upon solar activity in lands beneath the Tropics. 



These typical desert features have already been referred to, and 

 include — 



1. The sweeping of all fine material from the surfaces of the plains 

 by the action of the wind, and formation of plateau summits. 



2. Intense scouring of these surfaces by wind-driven sand. 



3. The breaking up of the most solid rocks by temperature variation. 



4. The formation of sand dunes beliind obstructions or where the 

 relief of the ground favours their development. 



5. The formation of mushroom-shaped pillars, or standing-out of 

 harder materials on bases undercut by the sand. 



6. The formation of sand-worn pebbles of typical angular outlines, 

 the well-known Dreikante. 



7. Vermicular markings on limestones, due it may be to etching 

 during the movement of evaporating saline solutions. 



8. Formation of desert-crusts by leaching out of the soluble 

 materials contained in the rocks, with evaporation at the surface, 

 resulting in deposition of the oxides of iron and manganese. 

 Mr. Lucas, Director of the Survey Department Laboratory, has made 

 a special study of these desert and river films, the latter probably 

 only differing from desert ones in degree. 



9. Flaking off of surfaces in the surface zone affected by tem- 

 perature variation. Also fracture due to the same cause. Fragments 

 of porphyry, limestone, etc., are often split into a series of parallel 

 flakes standing vertically, their original connexion to one another 

 being clearly indicated by their close juxtaposition. 



In the half-desert where rain, though brief, is intensely active 

 while it lasts, a series of interesting phenomena are presented : deep 

 caiion-like valleys, boulder- strewn gullies, saw-back ridges, parallel- 

 dyke country, saline marshes, dry waterfalls or steep precipices in the 

 valley-floors, great talus-slopes. 



(4) The Denudation of Arid Regions by Wind and "Water. By 

 Johannes Walthee, Professor of Geology and Palaeontology in 

 the University of Halle, Germany. 



EVERY climatic region is characterized by a different type of dis- 

 integration and denudation of soft or softened rock by the 

 agents of erosion. In the nival region a cover of snow protects the 

 surface of the earth during a long period of the year. 



In the humid zone and also in the equatorial pluvial region the 

 soil is overgrown by a network of roots and rootlets of millions of 

 plants, which bind together the small particles and protect them 

 against wind and running water. 



In arid regions, where the rain is not sufficient to form perennial 

 rivers, and where the vegetation forms isolated patches in the barren 

 country, every particle of soft or disintegrated rock is quickly taken 



