206 Dr. C. A. Raimi — Geology of Perim Island. 



nepheline/ a certain amount of potash and of silica must be removed, 

 and '35 of soda added with the chlorine, an operation not difficult for 

 heated water with a fair amount of sodium chloride in solution. To 

 obtain it from albite, only -20 of soda would have to be added with the 

 chlorine, but 2-35 of silica, a rather large amount, must be removed. 

 In the case of the Ice River rock (and not in this alone) I believe 

 the larger patches of sodalite to have mainly replaced nepheline. 

 At the same time I think it has, to some extent, also replaced 

 a soda-felspar ; small irregularly outlined bits of the latter, with 

 a rather ' residual ' aspect, are enclosed in the sodalite, especially 

 near to its outside ; the edge of the felspar also, where the two are 

 in contact, seems to be a little corroded.' It is also evident, from 

 its occurrence in the minute cracks, that some of the sodalite, 

 -especially that in the small dodecahedra, has been deposited from 

 a state of solution. So I believe the sodalite to be a mineral of 

 secondary origin in the principal cases mentioned in this paper, 

 without, however, disputing that in some others it may be an original 

 constituent. 



III. — Notes on the Geology of Perim Island.^ 

 By Cathekinb A. Raisin, D.Sc. 



[AM indebted to Mr. J. A. Rupert Jones (Lieut. R.N.R.), now at 

 Aden, for a series of rock specimens collected and sent from 

 Perim Island, accompanied by notes and diagrams. From all these 

 I have drawn up a short preliminary account, since they illustrate 

 some points of interest, and no complete description of the geology 

 of the island, as far as I know, has been published. 

 I. General Description. 

 Perim is a rocky barren island about 3 miles in length at the 

 entrance to the Red Sea, bounded on the east by the " small strait," 

 1^ miles broad, and on the west by the " larger strait," 9 to 10 miles 

 across. It is roughly of a horse-shoe shape, surrounding a harbour 

 opening to the south. Low plains below the 12 feet contour-line, 

 evidently raised beaches, form much of the island (perhaps one- 

 half or more), extending inland, especially at the north. The 

 neighbouring sea is shallow, the 5 fathom contour-line bending into 

 the harbour, the 10 fathom line extending outside the harbour to the 

 south and around a large submarine plateau to the north. A narrow 

 depression begins in the broader strait, and, after an interval, 

 recommences further north and extends almost the whole length of 

 the Red Sea, sometimes over 1,000 fathoms deep * — the whole 

 resembling a submerged valley with abrupt sides. 



1 Fouque & Levy (loc. cit.) remark that the chemical formula of sodalite (allowing 

 for the chlorine) is identical with that of nepheline. They and Rosenbusch (loc. cit.) 

 allow that it may be a secondary mineral. 



2 Morozewicz made sodalite artificially by fusing kaolin or nepheline with soda 

 and excess of sodium chloride ; see Journ. Chem. Soc, vol. Ixxvi, pt. 2 (1899), p. 764. 



3 An abstract of this paper was read at the British Association Meeting, Glasgow, 

 ,1901. 



* The above depths are taken from the Admiralty Chart. 



