Dr. W. F. Hume — On Walthers Desert Erosion. 21 



chemical changes of this nature was illustrated in the Graving Dock 

 at Alexandria. Certain curious coralloid-looking growths, formed 

 along fissure-planes in the bottom of the dock, proved on examination 

 in the Chemical Laboratory of the Survey Department to be 

 compounds of magnesium and calcium carbonates, produced by the 

 attack of the magnesium salts in the sea- water on the carbonate of 

 lime in the concrete blocks. 



On p. 128 et seq. the question of weathering under the influence 

 of solutions rising to the surface is carried a stage further. By the 

 crystallization of the salts fragments are split off the surface and, 

 where exposed, are soon broken up and carried away. This is not, 

 however, the case in cavities and wind-shaded spots. There the 

 loosened plate-like portions attached to the parent rock, and, if they 

 are hygroscopic, may contain sufficient water of solution still to 

 be chemically active. This affords another explanation for the 

 remarkable cavities developed in the granites of tropical regions, 

 where their position is such as to exclude the direct action of wind- 

 borne sand. It will require a chemical pronouncement to decide 

 whether the contained salts in the weathered portions would be 

 sufficient to produce the remarkable effects observed. 



Professor AYalther next considers the conditions which lead to 

 fracture of rock masses, such as is commonly assumed to result from 

 the influence of temperature variation. He illustrates the conditions 

 observed by photographs of the ' melon ' region of splintered flints in 

 the Libyan Desert, and especially by some fine photographs of 

 fractured granite and diorite blocks, the latter from the pass of 

 Belowitak in the Ethiopian Hills. Here the diorite possessed most 

 of the characteristic features of desert weathering. It has a deep 

 brown surface film, the individual blocks have been fractured asunder 

 into pieces whose former connexion is indisputable, the surfaces are 

 breaking up into flakes arranged tangentially. The rocks have 

 undergone no chemical alteration, but all around the foot of the huge 

 boulders is a coarse gravel composed of small fragments of the rock, 

 or where granite is present of small quartz and felspar grains. 



Each of these characteristics, which combined are the true * dry 

 weathering', has its special method of origin, the most easy to 

 explain being the tangential flaking, or desquamation (to use 

 von Kichthofen's term). When a boulder is warmed by day and 

 cooled by night the effect of increase of volume due to the heating 

 only extends to a small distance inward, the thickness of the surface 

 affected depending on the nature of the rock. Gebel Kassala is given 

 as an example of the remarkable smooth surface slopes covered with 

 liuge ' shells ' separated from the parent mass and still only slightly 

 attached to it, while around the base lie the shattered fragments 

 of others loosened from their supports. Fig. 70, illustrating this 

 feature, is somewhat disappointing. Plate xi in my Eastern Sinai 

 memoir is a far more impressive example of these occurrences. 

 Excellent illustrations are given of a fractured quartz grain and of 

 a fracture mosaic on p. 61, while in studying this section the reader 

 may turn back with advantage to figs. 2o and 26 for examples of 

 * pitted ', ' flaking ', and ' vertical ' splitting. 



