GEOLOGICAL HISTORY OF EGYPT AND THE NILE VALLEY. 311 
drain the western side of the Nile Valley for a distance of 
150 miles, as well as to irgate the Rayan and Fajim 
basins. Other hollows and oases are those of the Sittrah 
Sea, Uttiah, Aradj, and Siuah, some parts of which are lower 
than the Mediterranean. The origin of these remarkable 
depressions is somewhat difficult to explain. They are, it 
must be recollected, hollowed out of limestone strata, in 
which water acts as a solvent when containing carbonic acid 
gas. But their original formation doubtless dates back to 
the Pluvial period, to which we shall presently recur, when 
water action was vastly more effective over these regions 
than at the present day. 
3. The sandy desert, the most frightful of all desert sur- 
faces, is covered by mounds, sometimes 100 métres high, of 
pure quartz sand. The source of this sand, which often 
covers limestone strata on the borders of the Sahara, 
was, according to Zittel, probably the “ Nubian sandstone 
formation,” which underlies the limestone, and which has 
been drifted from its outcrop by the winds northwards 
from the Oasis of Chargeh, and westwards along the 
parallel of lat. 25°. Doubtless, also, some of this sand 
may have constituted the sandy bed of the sea, when 
the whole region of northern Africa was gradually rising 
from beneath the waters, in later Tertiary times, and exten- 
sive denudation of the strata was in progress.* 
Forest of Silicified Wood.—Lying in a direction due west of 
Cairo, lat. 30, and ina region but little explored, are to be 
found great numbers of silicified stems of trees, apparently 
belonging to a sandstone deposit, and extending over a tract 
1,600 square miles. This forest is formed of the stems of 
trees of the genus Nicholia, &c., and has its counterpart in a 
similar forest on the plateau of Jebel Ahmar, near Cairo, 
where the trees have been embedded in reddish loam and 
variegated sandstone or conglomerate. This deposit is 
generally regarded as of Miocene age.t 
* Professor Judd states that the sands of the alluvial deposits of 
Lower Egypt give evidence of having been derived originally from 
granitoid rocks, as the grains contain fluid-cavities and crystals of rutile 
seen under the microscope. A similar observation has been made by 
Dr. J. 8. Hyland as regards the grains of quartz in the sands at Korti, 
which contain gas- and fluid-cavities as well as hair-like needles, pre- 
sumably of rutile. Scient. Proce. Roy. Dublin Soc., Feb. 10, 1890. 
+ For fuller accounts of this deposit, see Dawson, Modern Science in 
Bible Lands, p. 543. Zittel, Libyschen Wiiste, p. 132. Carruthers Geol. 
Mag., 1870. 
