Vol. 67.] EASTERN DESERT OE EGYPT. 261 



Plate XVI. 



Fig. 1. Block of granite from the Urn Esh range, showing the fretting-away of 

 the felspar, the outstanding quartz, also the sand-grains driven into 

 the softer material and lodged between the quartz-grains. (Two- 

 thirds of the natural diameter.) 

 '2. Arrowhead twin of gypsum from Jubal Island, showing the mecl 

 cally etched figures. (Four-fifths of the natural diameter.) 



umi- 



Discussion. 



Dr. C. G. Cullis suggested a special determination of the crystal- 

 rhombs described, in order to determine whether they were really 

 dolomite, and not calcite. 



Sir Thomas Holland supported the previous speaker's suggestion 

 for a special check determination of the rhombohedral crystals of 

 supposed dolomite. Beautifully sharp rhombs are found in the 

 black mud of the llajputana salt-lakes, but on chemical examination 

 they prove to be calcite, not dolomite. Recent work in the 

 llajputana Desert reveals many features described by the Author, 

 and long recognized as characteristic of the Triassic and other 

 salt-bearing formations : among these are the plano-convex shape of 

 the lenticular deposits ; the contemporaneous formation of gypsum 

 and dolomite with the salt; and the distribution of pebbles in an 

 abundant sandy matrix, due to the action of sudden floods in desert 

 regions. The work done by the speaker in conjunction with 

 Dr. Christie (Eec. Geol. Surv. Ind. vol. xxxviii, 1909, p. 154) 

 showed that the salt, of which over 54 million tons exist in Sambhar 

 alone, is brought in in the form of fine dust from the coast of Cutch, 

 some 500 miles away to the south-south-west. With the fine 

 angular sand of the JBikaner Desert are found undamaged tests of 

 foraminifera, which must have been carried bodily by the wind 

 instead of being rolled with the heavier rounded sand-grains. 



Dr. J. \V. Evans was glad to find that the Author employed the 

 term dreikanter for the normal type of wind-worn stone with 

 three approximately parallel edges, and not for the more special 

 roughly tetrahedral form to which it was applied by some authors. 

 He was much impressed by the evidence brought forward of the 

 destruction of rocks by wind-action. He believed that many of 

 the so-called ' plains of marine denudation ' would prove to be the 

 result of wind-erosion, aud instanced that on which the Cambrian 

 of the Xorth-West of Scotland was deposited, as also the planed- 

 down surface of the Palaeozoic strata in South Wales and elsewhere, 

 which was believed to date from the Triassic Period. 



The Prksidknt (Prof. Watts) regretted that time did not permit 

 of adequate discussion of the numerous and interesting points raised 

 in the paper. The Author had spent a very considerable time on 

 the ground and in working up his conclusions, and there could be 

 no doubt that the paper constituted an important addition to the 

 knowledge of desert conditions and deposits. 



The Author thanked the President and the Fellows for the way 



