1885.] Report on the Deposits of the Nile Delta. 



215 



Embedding these grains in Canada-balsam, and examining them 

 by transmitted light, with the aid of the polariscope, we are enabled 

 to study their mineral characters with facility. The majority of the 

 grains consist of colourless quartz, though occasionally rose-quartz, 

 amethystine quartz, citrine, and smoky quartz also occur. This 

 quartz exhibits unmistakable evidence of having been derived from 

 granitic rocks ; it is constantly seen to be traversed by bands of 

 liquid- and gas-cavities, and very frequently contains numerous black, 

 hair-like inclusions (rutile?). Much more rarely we detect grains of 

 quartz which consist of aggregates of small crystals, and are evidently 

 derived from metamorphic rocks. With the pure quartz-grains we find 

 also a considerable number of rounded particles of red and brown jasper 

 and of black Lydian stone, with some fragments of silicified wood. 



But in addition to the different varieties of quartz, particles of 

 felspar are sometimes found among these large, rounded grains. 

 What is very remarkable about these felspar-grains is the slight 

 traces of kaolinization which they exhibit ; they are in fact almost as 

 fresh and unaltered as the grains of quartz themselves. Ordinary 

 orthoclase and microcline are most frequent, while plagioclase felspar 

 is comparatively rare. With the rounded grains of quartz and 

 felspar, a few examples of hornblende and other minerals, including 

 jade, also occur. 



But far greater is the number of mineral species, especially the 

 most easily cleavable ones, which are represented in th.e smaller, sub- 

 angular and angular, sand-grains. In addition to the minerals already 

 mentioned, I have recognised several varieties of mica, augite, 

 enstatite ?, tourmaline, sphene, iolite (cordierite) zircon, fluorspar, 

 and magnetite, all in a nearly unaltered condition. 



The only fossils found in these sands were evidently derived ones ; 

 they include the fragments of silicified wood already mentioned, and 

 a waterworn nucleus of an Ammonites (Jurassic ?). It is evident that 

 these sand-grains have been formed by the breaking up of granitic 

 and metamorphic rocks, or of older sandstones derived directly from 

 such rocks. The larger grains exhibit the perfect rounding and 

 polishing now recognised as characteristic of iEolian action ;* the 

 smaller ones from their larger surfaces in proportion to their weight 

 have undergone far less attrition in their passage through the air ; but 

 it is fair to conclude that the sands are really " desert-sand," derived 

 from the vast tracts which lie on either side of the Nile-Valley, and 

 swept into it by the action of the wind. 



* It is hardly necessary to point out that the study of these desert-sands entirely 

 supports the important conclusions arrived at hy Mr. H. C. Sorby, F.R.S., and 

 Mr. J. A. Phillips, F.R.S., concerning the agencies by which the rounding of sand- 

 grains is effected. See " Quart. Jour. G-eol. Soc," vol. 36 (1SS0). " Proceedings," 

 p. 50. Ibid., vol. 37 (1881), pp. 6—26. 



