Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/101

 350 to 400 feet more. The top of the hill is capped by a thin limestone, from 8 to 15 feet in thickness, which is tolerably persistent on the northern side of the district. It is usually grey or light brown, very crystalline, and at times much resembling spathic iron-ore. It contains a few obscure fossils, chiefly encrinite stems and rings, with a few imperfect sections of cups ; and, according to Mr. Etheridge, these are to be referred to Triassic forms. Mr. Salter, on the other hand, states that they are Carboniferous, including forms of the genera Rhodocrinus and Poteriocrinus, and that they are associated with the spiral univalves Murchisonia and possibly Eulima.

The thin limestone-beds are covered by other sandstones similarly false-bedded to those below, but of a lighter colour, and they form the highest rocks in the Nasb district ; but further south, in "Wady Nagb el Bedra, they are succeeded by beds of Cretaceous age.

Age of the Sandstone. — The same series of sandstones has been described by other geologists as occurring in Egypt and Nubia under similar conditions, i. e. between older crystalline schists below and undoubted Cretaceous beds above. Russegger calls it " Nubian Sandstone," colouring and describing it as Lower Cretaceous in his maps of Egypt, Nubia, and Arabia Petraea. The text, however (Reisen, vol. ii. pt. 1. p. 570), contains no positive statement to the above effect, but only says that the beds in question are not younger than the Lower-Cretaceous period.

Figari Bey describes the tripartite arrangement of the series (two great masses of sandstone, with a thin limestone between them) as occurring in Egypt, Sinai, and in the neighbourhood of Akaba ; and in the latter place he states the total thickness to be about 850 feet *. He assigns the whole to the Trias, taking the limestone as representing the Muschelkalk, although the evidence for this determination (other than lithological character) is not very clear. The only fossil mentioned is a good-sized Ammonite, not named specifically, but said to approximate in character to a Triassic form†. I have conformed to this view, without, however, wishing to express any very decided opinion on the subject, owing to the great want of evidence.

Manganese-bed of Nash. — The thin limestones capping the hill are, on the western side of Wady Nasb, thrown down to the bed of the watercourse by a fault running nearly north and south. On both sides they are commonly associated with ores of iron and manganese, which run in a nearly continuous bed of varying thickness, as a chain of pockets or lenticular deposits, over the greater part of the district. The mineral usually consists of a compact brown haematite, passing, by admixture of quartz, into an iron -jasper or extremely ferruginous sandstone. Some of the larger beds, which are from 5 to 8 feet thick, contain compact and mammillated brown haematite in large masses, with small, but beautifully transparent, crystals of gothite in the hollows. Pyrolusite is also found in considerable quantities in places, usually in large nodular masses, enclosed in the iron-ore and associated with psilomelane. The base of the cliff, throughout


 * Studii scientifici sull' Egitto e sue adjacenze, tom. ii. p. 550.

† Ibid. tom. i. p. 146.