ON USEFUL AND ORNAMENTAL STONES OP ANCIENT EGYPT. 275 



peculiar rugged and dark-coloured appearance attracts, more 

 or less, the attention of all travellers, who have usually 

 regarded it as of volcanic origin. Geologists, as Russegger, 

 Newbold, Schimper, Fraas, Delesse, Schwemfurth, and Owen, 

 have naturally given attention to it, and have discussed its 

 relation to the fossil wood of the so-called petrified forests in 

 its vicinity. 



Stratigraphically it consists of beds of more or less indu- 

 rated siliceous sandstone resting on the Upper Eocene lime- 

 stones of the Mokattam hill, but differing entirely from 

 them in appearance and mineral character. The stratigraphy 

 thus proves that these sandstones are newer than the Eocene, 

 and they have usually been regarded as of Miocene age, so 

 that we have here an example of an intensely indurated rock 

 of comparatively modern date. Quite recently Mayer-Eimar 

 has, on the ground of certain fresh-water shells found in con- 

 nection with these beds, assigned them to the Tongrian, or 

 Lowest Miocene age,* and with this view the evidence of the 

 fossil trees is sufficiently in harmony. Of the older autho- 

 rities, Russegger and Newbold seem to have very clearly 

 understood the character and relations of these singular 

 deposits. In point of fact, Jebel Ahmar, and some neigh- 

 bouring eminences of similar character, constitute the unde- 

 nuded remnants of thick beds of sandstone once spread 

 uniformly over this region on both sides of the Nile, and 

 deposited in shallow water succeeding the deeper water in 

 which the Eocene limestones were laid down. Into this 

 shallow water drifted many trunks of trees, principally of the 

 genus Nicolia, and other exogenous trees believed to be allied 

 to certain modern species of interior Africa. f With these are 

 trunks of palms, and of Coniferous trees allied to the yew. 

 The wood was silicified, and the sandstone in places 

 hardened into quartzite by the percolation of siliceous 

 waters. The action of the sea and of atmospheric agencies 

 in later Tertiary times have removed the less consolidated 

 portions, leaving the silicified trees scattered about, while 

 there remained as rugged eminences those portions of the 

 beds which had been hardened into quartzite by siliceous 

 infiltration. 



That this is the origin of these hills is evident from the 



* Bulletin Zurich Academy, 1889. 



t R. Brown, Quart. Jovrn. Geol. Soc, iv. Carrnthers, Geol. Mag., vii. 

 Scheuk, in Zittel's Lybischen Wuste. One of the Conifers in my collection 

 is a Taxites of modern aspect. 



