334 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
intermixture with other substances, and their consequent opacity, 
giving rise to dendritic and other delineations. It seems, he goes on 
to say, as if the solution and conversion of the organic into the inor- 
ganic in the Egyptian pebbles, is throughout more perfect than it is 
in many flints, although the constituent elements of both kinds of 
stone are very probably quite the same. 
Previous to passing to a notice of the organic contents of the lime- 
stone, I may briefly notice some singular siliceous bodies that occa- 
sionally occur imbedded in it, and are particularly numerous in the 
limestone rocks of Thebes on the Libyan bank of the Nile. They 
cover the debris at the foot of the cliffs in such profusion as to 
be termed by the Arabs nuktah, or drops, which they suppose 
to have been rained from heaven. They are also seen there in situ, 
disposed conformably in a horizontal layer of whitish marl in the 
earthy white limestone, which abounds with thin seams of crystallized 
gypsum, muriate of soda, and cale spar. These bodies usually assume 
the shape of spheroids encircled by a belt, resembling the delmeations 
of a planet with its belt; two are sometimes connected together, 
while others assume various modifications of form. They have a 
thin whitish coating, and in the interior present a greyish or brownish 
chert, like the ordinary nodular chert already described. Ehrenberg*, 
who has lately examined these siliceous spheroids, terms them ocel- . 
lated stones, or morpholites ; he found no traces of organic structure, 
and is of opinion that they are the result of a crystalloidal or morpho- 
litic force. Their structure does not present radiation from the centre; 
nor any appreciable crystalline development in their parallel planes of 
formation founded on uniform laws, which frequently, perhaps always, 
parted from many axes of formation. In the curious structure of 
these bodies Ehrenberg discovered foreign bodies, small stones, frag- 
ments of granite, &c. 
Economical Uses of the Limestone.—It is remarkable that most of 
the earliest monuments of Egypt reared by human hands should be 
composed of this limestone, a formation, geologically speaking, but of 
yesterday. Most of the older temples, grottos, and tombs in Central 
and Lower Egypt—the great Sphimx and the Pyramids themselves— 
were formed of it. Soft and sectile, like the Portland stone, in the 
quarry, it hardens rapidly on exposure to the air and the sun’s rays. 
Upper or Overlying Sandstone (fig. 1,6; fig. 2, a).—A sandstone 
formation, associated with calcareous, gypseous, and saline marls, in 
horizontal layers, overlies the limestone just described in detached 
hummocks and patches stretching from the Mediterranean far into 
the Nubian and Libyan deserts, and has been traced into Abyssinia f. 
The discontinuance of its beds has evidently been caused by denu- 
dation, the softer portions having been swept away, and the debris 
seattered over the desert; while the more consolidated beds are left 
standing, as at Gebel Ahmar, near the petrified forest near Cairo, 
and are washed into abrupt, irregular, and fantastic shapes. 
Lithological Character.—The sandstone varies from a compact ery- 
stalline rock of a blood-red, white or yellow colour, to a loose quartzose 
* Edin. Phil. Journal for April 1841, pp. 356, 357. + Lefevre: 
i 
