NEWBOLD ON THE GEOLOGY OF EGYPT. 335 
erit and conglomerate, imbedding rounded and angular pebbles usu- 
ally of a siliceous nature, viz. quartz, chert and jasper, and derived 
principally from the subjacent limestone. The cement agglutinating 
the grit is usually siliceous and ferruginous, mixed with decayed 
felspar, and sometimes lime. In many localities it imbeds silicified 
trunks and fragments of trees, particularly near Gebel Ahmar near 
Cairo, and Wadi Ansari, about eight hours’ journey to the eastward. 
In this vicinity I observed in it casts of pelagic shells. Lefevre has 
traced it into the Baytida desert, where it also contains silicified 
wood, and is composed of grains of quartz and rounded fragments of 
the sandstone, and felspathic rocks on which it rests. 
The specimens of wood which I brought from the locality near 
Cairo were kindly examined, at my request, by Mr. Robert Brown, 
and all those which could be determined were pronounced to be di- 
cotyledonous, and not coniferous; but a specimen of the latter was 
brought from Abusambel, by the Rev. Vere Monro* ; and I observed 
another, of a reed, in the collection of Mr. R. Brown. A more de- 
tailed account of the fossil wood will be given in a subsequent part 
of this paper. | 
The beds of this sandstone are usually very thin, varying from a 
few inches to 180 or 200 feet in thickness. The associated marl- 
beds rarely exceed ten feet, and are often much thinner. They are 
of various shades of white, brown, and green ; and, as before stated, 
are of an argillaceous, calcareous, and. gypseous character. Those 
covering the great platform of the Libyan desert, from the Mediter- 
ranean to the Oasis of Ammon, are said by Ehrenberg+ to contain 
known tertiary forms. They enclose layers of crystallized carbonate 
and sulphate of lime ; the latter, as in European rock-salt formations, 
is associated with muriate of soda. These substances, acted upon by 
water and the atmosphere, afford the necessary chemical conditions 
for the natural production of muriate of lime, carbonate and sulphate 
of soda, which are found in the natron lakes that occur in this forma- 
tion. The most celebrated are the six lagunes on the northern and 
eastern flank of the Waterless river, about fifty miles ina direct line 
south from Alexandria, and twenty-six from the western branch of 
the Nile. They present a chain of shallow pools, formed by water 
which percolates through the marl and sands on their banks, and is 
usually tinged of a red colour by a substance of which an accurate 
analysis is desirable. It is supposed to be of a vegeto-animal nature. 
The red water, in the hot season, has a highly fetid, ammoniacal 
odour, and a caustic alkaline taste. As the water evaporates, during 
the hot season, the newly-formed salts (except the portions that re- 
main held im solution) are deposited in incrustations on the sides and 
banks of the lagunes, and collected. 
From the circumstance of the supply of water in the natron lagunes 
varying with that in the Nile, it has been supposed that their beds 
are at a lower level than the river, and even below the surface-level of 
the Mediterranean, a fact however still to be ascertained. 
* Edin. Phil. Journal, vol. xviii. p. 337. 
+ London Phil. Mag. June 1841, p. 445. 
