Vol. 67.] E\ STERN DESERT OF EGYPT. 249 



constituents. A specimen of the granite-surface which 1 obtained 

 shows this action in a remarkable manner, the quartz - grains 

 standing out in high relief, and the felspars being almost entirely 

 eaten away. The flying sand-grains have been driven on to the 

 softer portions of the rock with such violence, that they are to be 

 seen still wedged firmly into every crevice (see PI. XVI, fig. 1). 



This, however, is not the only way in Avhich the sand assists 

 in the disintegration of the igneous rocks. There is still another 

 phenomenon, which does not appear to have been noted hitherto. 

 The very finest material is forced into the tiniest cracks and crevices 

 in the rocks. Pocks, which at first sight appear to be quite sound, 

 crumble to pieces when struck by the hammer, and all along the 

 shatter-planes will be found this very fine sand penetrating to 

 considerable depths. Here it tends to assist the other agents of 

 disintegration : the action of frost, the alternations of heat and cold, 

 the percolation of moisture, and more rarely the vital forces due 

 to the action of plants and animals. The grains act as tiny wedges, 

 slipping ever farther into the cracks when opportunities occur, until 

 finally the parts are so forced asunder that the rock falls to pieces. 



The andesites and porphyries become more polished and grooved 

 than the granites, and show less tendency to crumble. They also 

 are more liable to develop facets where the blowing sand-grains 

 have planed off corners and edges. Sometimes these rocks are 

 grooved all over in a peculiar manner by the action of the wind- 

 blown sand, and resemble exactly the specimens obtained by Prof. 

 AVatts l from the Triassic rocks of the Charnwood-Forest area. In 

 some specimens, hard zeolites have resisted the action of the wind 

 and stand out all over the surface, capping curious protuberances 

 which look very like collar-studs. 2 



One of the most interesting results of the action of wind-blown 

 sand was observed on Jubal Island. This Avas the mechanical 

 etching of the crystalline gypsum (selenite) which occurs there. The 

 calcareous sand wears away the faces of the crystals, but its action 

 is selective. The greatest wearing effect seems to be produced in 

 directions parallel to the orthopinacoid (1.0.0.) and the negative 

 hemipyramid (1.1.1.). The result is a beautiful series of etched 

 figures produced mechanically, covering the clinopinacoidal faces of 

 the mineral (sec PI. XV I, fig. 2). 



Other phenomena, due to Avind-blown sand, such as the formation 

 of dreikanter, have been dealt with elsewhere.' The action of 

 the sun upon the flints and cherts derived from the Eocene lime- 

 stones is worthy of note. The surfaces of such flints are usually 

 chipped all over, owing to small fragments having broken off with 

 a conchoid al fracture, and sometimes the fracture has left perfect 

 cones on the surfaces. In such cases, the angle of slope of the cone 

 was usually found to lie between 30° and 35°. 



1 W. W. Watts, Ecp. Brit. Assoc. 1899 (Dover) p. 747. 



2 Mr. T. O. Bosworth has also used this simile, in describing comparable 

 occurrences on the coast of Mull, Geol. Mag. dec. 5, vol. vii 1 1910) p. 354. 



3 Geol. Mag. dec. 5, vol. vii (1910) p. 394. 



