PKOCEEDINGS OE GEOLOGICAL SOCIETIES. 



465 



"On the Geology of the Egyptian Desert." By Mr. H. Duckworth, 

 F.G.S. — Egypt, with its desert tracts, covers an area of about 100,000 

 square miles. In form it is an irregular parellelogram, the longest axis of 

 which runs north and south. It is that desert tract, east of the Nile, 

 which forms the subject of this communication, generally known as the 

 " Egyptain Desert," and which, from its connection with the Overland 

 Route and the Suez Canal scheme, has had more than an ordinary amount 

 of attention directed to it. The desert which stretches between Cairo and 

 Suez is more varied in aspect than those who are acquainted with it might 

 be disposed to believe, the ground being everywhere broken up into a series 

 of undulating tracts, or shallow ravines, called by the Arabs " Wadis," 

 whilst the mountain ranges of Jebel Mukattam, Jebel Reibun, and Jebel 

 Attakah, form a complete barrier towards the south. The surface of the 

 land presents generally the appearance of a gravelly beach, pebbles and 

 angular fragments of jasper, chert, quartz, gypsum, and sandstone being 

 largely intermingled with sand. V egetation is not altogether absent, and 

 in the Wadis several ligneous and herbaceous plants are found, among 

 which are the Acacia JEgyptiaca, the Astragalus hamosus, and the Fa- 

 gonia latifolia (Sieberi et tumidus). The height of the desert varies con- 

 siderably, but its extreme elevation above the sea may be stated at about 

 700 feet. Its general character is that of an elevated plateau, rising towards 

 the centre and gradually sinking until it terminates in bold escarpments 

 on the banks of the Nile on one side, and on the shores of the Red Sea on 

 the other. These cliffs are composed for the most part of Nummulitic 

 limestone, which we find extensively developed throughout Egypt and 

 Nubia and a great part of the Sinaitic peninsula. Overlying this limestone 

 are patches of a sandstone formation, associated with calcareous gypsums 

 and saline maris, and stretching from the Mediterranean Sea far into the 

 Nubian and Libyan deserts. The absence of its beds at certain points 

 1 has evidently been caused by denudation, and the sands and gravels of the 

 desert may be regarded as the debris of this formation. The beds of this 

 H sandstone vary in thickness from a few inches to 180 to 200 feet. Siliei- 

 fied monocotyledonous wood is found in great abundance in this deposit, 

 especially in the vicinity of Cairo, where the remains are so well preserved, 

 and of such magnitude, as to be popularly known as the " Petrified Forest." 

 The wood-stems in question are invariably found in a horizontal position, 



i and there is no evidence to show that they originally flourished in situ. 

 Professor Unger, of Vienna, states that the trees belong, without excep- 

 tion, to one species, for which he proposes the provisional name Nicolia 



, JEgyptiaca ; and further, that the sandstone in which they occur is strictly 

 analogous to a formation containing wood-stems near Gleischenburg, in 

 Styria. He supposes the masses of wood to have been drifted into a basin 



i separated from the main sea, and filled with water saturated with silica, — 

 an hypothesis which appears to me extremely probable. 



" On the Composition of Black Sandstone occurring in the Trias around 

 Liverpool." By Mr. A. Norman Tate, F.C.S. — These deposits are found 



I in the sandstone of Flay brick, Storeton, Toxteth Park, and other places. 

 The author's attention was directed to them by Mr. Morton, and he has 



; since examined them chemically to ascertain their composition. On treat- 



, ing them with hydrochloric acid, chlorine gas was evoived, and the dark- 

 coloured portion dissolved, leaving a residue of white sand. A quantitative 

 analysis showed that, next to silica, the principal ingredient was peroxide 



j of manganese. One sample contained as much as 10 per cent, of that sub- 



■ stance, whilst in others the quantity did not exceed 35 per cent. To the 

 presence of this peroxide of manganese the black colour is evidently due. 



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