Dr. W. F. Hume — On Walthers Desert Erosion. 73 



extension of the glaciers occurred some time after the time of lowest 

 temperature, and the Scandinavian ice-sheet may liave reached its 

 greatest extension when the British ice was melting away. 



Britain, being near the edge of the continental shelf, would 

 respond more readily to temperature changes than the Scandinavian 

 ice-sheet to the north-east. It may therefore have happened, as 

 has been suggested above, that the British ice was on the wane long 

 before the Scandinavian ice reached its greatest extension. 



Temporary changes of temperature would also tend to affect the 

 margins of the ice-sheet rather than the centre of the area of dis- 

 persion. It thus seems to have come about that the maximum 

 extension of different portions of the ice-margin occurred at different 

 times, and that temporary' slight ameliorations of climate affected the 

 nuirgins of the ice-sheet supplied by the British mountains more than 

 they did the marginal portions fed by the Scandinavian ice. 



If this reading of what took place be correct, we must not regard 

 a map showing the ground which had been covered by the ice-sheet 

 at maximum extension as indicating the actual extent of the ground 

 covered by ice at any one time, for it would appear that when 

 the North Sea ice reached the neighbourhood of London the glaciers 

 from the British mountains were already on the wane. 



VI. — Pkofessoe Walther's Erosion in the Desert considered. 



By W. F. Hume, D.Sc, F.E.S.E., F.G.S., 

 Director of the Geological Survey of Egypt. 



(Concluded from the January Number, p. 22.) 



SECTIOiST 15 deals with the very interesting subjects of desert films 

 and crusts. Two features are very noticeable in many desert 

 limestone rocks, the production of a hard crust on the outside and of 

 a powdery soft interior. The outer hard layer is often of different 

 colour from the internal portion and highly polished. Professor 

 Walther points out that such hardened crusts occur in buildings in 

 Central Europe, where he suggests that during the dressing of the 

 stone the fine powder produced has been so driven into the pores that 

 chemical agencies are no longer as active. I ventured a suggestion for 

 the production of such sub-crj-stalline surfaces where much exposed to 

 the sand-blast in which action such as the above was vaguely present 

 to my mind, but whatever external physical effect such external 

 activities may have, the more important is undoubtedly the con- 

 centration of solutions near the surface under the influence of the 

 solar heat. 



Where erosion along cracks has been of long duration the hard 

 layers may alone be left, giving rise to various gnarled and contorted 

 structures in the rocks which are a characteristic feature in desert 

 limestone weathering. 



In a brilliant word-picture (p. 142) Professor Walther draws the 

 contrast between a Euro])ean and a desert landscape. In the first 

 case a broad covering of green gives the fundamental colour tone. 

 In the desert scene, on the contrary, yellows and browns in every 

 shade from delicate golden yellow to the deepest coffee-brown or 



