Sa ew, oe 
~ 
354 ‘PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
this deposit and the silicification of the trees, its upper and looser 
layers were removed by denuding aqueous action, evidence of which 
will be adduced hereafter: the trunks, thus left unsupported, either 
fell by their own weight increased. by silicification, or were laid pro- 
strate by the action of the current, and their smaller fragments, min- 
gled with sand and gravel-detritus of the subjacent rock, scattered to 
considerable distances, contributing to the formation of the present 
surface of the desert. 
The imbedded fossil stems are rarely flattened, and do not bear any 
vestiges of ever having been covered with the thin coating of coal we 
see envelopmg some of the trunks of Dixon Fold,—a circumstance 
easily explained when we consider the more perfect process of silicifi- 
cation to which the former have been subjected, exhibitig often the 
finest anatomical structure of the interior with a perfection equalling 
that of the tree in a state of nature, converted into silex, and rivalling 
' oriental agate and cornelian mm transparency and colour. Mr. Robert 
Brown has kindly examined the specimens I brought with me from 
Egypt, and reports that the three whose characters he could deter- 
mine are dicotyledonous and not coniferous*. 
Geognostic position.—The basis on which repose the strata of sand 
and pudding-stone imbedding the fossil trunks, and indeed that of the 
whole of this portion of the Egyptian desert, is the ordinary marine — 
limestone of the Mokattem in nearly horizontal beds having a seareely 
perceptible westerly dip. The inferior beds (6, fig. 8) of the lime- 
stone in this vicinity have a chalk-like colour and texture, imbedding 
nodules of brown, grey and blackish chert, covered by a thin white 
coating in regular layers. The Fig. 8. 
upper beds (5) are more compact, 
interstratified with thin layers (from | °- - 
2 to 12 inches in thickness) of a }|——~——— 
dull greenish gypseous and salife- 
reus' marl (a, A,’5), and contain, | ——=——_— ae 
among other pelagic remains, num- ae 4 
veddlica mee ae peers corallines, H sul Hl 
fishes’ teeth, [these beds have been ; : 
referred by some French geologists |... 2.2.2. Be i 
to the chalk period.| (4) is a thin 
argillaceous bed varying in colour - 
from red to a dull green. (3) is the eee 
sand and pudding-stone stratum im- 
bedding the petrifactions. On if, |= " 
5 
in some positions, rests’ a bed of | | ==) 35) Sassi 
argillaceous and gypseous marl with 
rock-salt (2), underlying the sand 
- — <_ _— — ae 
and gravelof the desert (1). This | = = lf 5-2] = 
bed however is generally wanting, | —_- ——__ —__ —— |6 
having been carried away by aque- ‘2s ee 
ous action. 
* A specimen of coniferous wood has been brought from the Nubian desert by 
the Rev. Vere Monro, found in a deposit analogous to that near Cairo, which is 
