NEWBOLD ON THE GEOLOGY OF EGYPT. 339 
Bryozoa, or coral animalcules ; which, though very small,—resem- 
bling grains of sand,—are yet, for the most part, larger than the chalk 
animalcules. Those singular beings constitute a large portion of the 
sand of the Libyan desert, and may perhaps be regarded,—should 
they be distinct, which M. Ehrenberg seems to suppose, from the 
animalcules of the calcareous rocks on which the sands repose,—as 
additional proofs of the submarine origin of the sands and gravel of 
the desert. 
Volcanic Rocks.—Though the existence of thermal springs,—some 
of them containing sulphuretted hydrogen,—the petroleum wells and 
sulphur deposits of Ezzeit, E-gimseh, and Gebel Kebrit, denote the 
continuance of igneous action below the surface, in a line following the 
volcanic belt of the Red Sea, indicated by the outbursting of Gebel Teer 
and Aden, the absence of earthquakes proves that, within the historic 
period at least, this energy has not been exerted with violence. 
M. Rochet d’Hericourt* informs us that he has noticed two old 
extinct volcanos in the desert between Cairo and Suez, and some small 
upheaved mounds of volcanic productions near Gebel Ahmar, on the 
Suez route: and Mr. St. John*+ states, that in the vicinity of Dakkeh, 
about 69 miles from Syene, in the Nubian desert on the left bank of 
the Nile, are numberless black cones, some higher than Vesuvius, 
supposed to be extinct volcanos, and covered with cinders and lava. 
But as yet our information of the nature and relations of these sup- 
posed volcanic tracts is of a meagre and undecided character. 
Trap Dykes.—Dykes of augitic trap of the variety termed dolerite, 
sometimes imbedding iron pyrites, are seen within and on the borders 
of the plutonic and metamorphic area of Upper Egypt, penetrating 
all the rocks from the lower sandstone to the granite. The overlying 
sandstone is undisturbed, and sometimes contains imbedded pebbles 
of the trap, the relative age of which is hence determined. Its in- 
trusion among the lower sandstone beds and clay-slate is marked near 
the junction-line by their conversion into jasper and jaspideous rock; 
and among the limestone rocks, by the formation of chert and agate, 
and a general tendency to silicification. Serpentines passing into verde 
antique are met with in the area just mentioned ; but may rather be 
classed here with the hypogene series, to which they are confined, 
than with the trappean. Porphyritic dykes traverse the granite, 
consisting of felspar crystals in a reddish felspathic base. A black 
variety also occurs resembling melaphyre, its base being black augite 
with crystals of felspar. Between the plain and pass of Abu Zeyran 
between Kossier and Thebes, I observed dykes of a rock almost 
entirely composed of red felspar, imbedding dark brown shining cry- 
stals, penetrating and overlying a boss of granite, as shown in the 
annexed diagram, fig. 4. 
Fig. 4. 
A. Granite boss. _ B. Felspathic rock. 
* Proc. Geol. Soc. of London, June 14th, 1841. t+ Travels in Egypt. 
