446 Scientific Proceedings, Royal Dublin Society. 



made up of two kinds of grains, the larger being perfectly rounded 

 and polished, whilst the smaller are often sub-angular or angular. 

 The grains often exhibit traces of deposits of iron oxides upon 

 their surfaces. The material constituting the sand is quartz — 

 felspar being extremely rare. The quartz shows every evidence 

 of having been derived from granitic rocks : it is very full of 

 gas- and fluid- cavities, and is often traversed by fine, hair-like 

 needles of a mineral which is presumably r utile. These are 

 arranged along definite planes, and cross each other at an angle 

 of about 60°. The latter phenomenon is a very characteristic 

 and constant feature of the quartz of granitic rocks. 1 Evidence 

 of the presence of quartz, such as finds its home amongst meta- 

 morphic rocks, is not wanting. In such cases the quartz is seen 

 under crossed nicols, to consist of aggregates of small grains — 

 in short, the quartz has undergone granulation. Zircon is to be 

 observed amongst the sand, but is not of frequent occurrence. 

 There are also some dark-coloured opaque grains which are pre- 

 sumably of a siliceous nature. But a few of the almost perfectly 

 spherical grains appear to be of calcareous origin. 



It is to be concluded that the desert sand had been formed by 

 the breaking up of granitic or metamorphic rocks, or of sedimen- 

 tary deposits directly derived from these rocks. Of still greater 

 interest is, however, the fact, enunciated so ably by Professor 

 Judd, that the fragments of felspars and other compound silicates 

 of the delta-deposits exhibit but slight evidence of kaolinization or 

 other chemical alteration. This is also true of the few felspars in 

 the Korti sands. The fact points to the conclusion that, in the 

 arid districts drained by the Nile, the disintegration of rocks is 

 effected by mechanical rather than by chemical agencies. 2 The 

 most potent of these agencies of disintegration are, according to 



1 H. C. Sorby has also described the sand of the Egyptian desert in his Paner 

 entitled " The Application of the Microscope to Geology : Micro. Journ., xvii. pp. 113- 

 136. 



3 It is a well-known fact that rocks are singnlarly well preserved in Egypt. 

 According to Newbold (I. c, p. 340), the granite of Egypt, 'for instance, is freer from 

 the decay, the " maladie du granite," than that of India. This probably arises from 

 the peculiarly dry atmosphere of Egypt, which has been mainly instrumental in pre- 

 serving, almost in their original freshness, its magnificent sculptures and vivid frescoes. 

 (See also E. Hull, On building and Ornamental Stones : London, 1872.) 



