Dr. C. A. Raisin — Geology of Perim Island. 207 



The islands in the Red Sea and the hills beyond the low coast 

 plains are sometimes of sedimentary strata (e.g. Nubian Sandstone), 

 sometimes of volcanic rock (as in Ketumbul, Jebel Teir). Other 

 hills are of Archsean gneiss, while many islands ai'e coral reefs or 

 formed of coral, and calcareous sediment must be often deposited.^ 



Above the 12 feet contour-line, the island of Perim is hilly, rising 

 to 249 feet at the highest point. The foundation rocks of the island 

 are exposed in cliffs and quarries, and all the specimens sent are 

 volcanic. The surface of the island, to a depth generally of about 

 7 feet, consists of irregular blocks, sometimes as much as 4 feet in 

 diameter, imbedded usually in whitish calcareous sand or mud, and 

 coral is common in the adjacent sea." As described by Mr. Rupert 

 Jones, the surface of the island has only to be rolled down when 

 damp (the big blocks being removed) to set into a hard, smooth 

 cycle track. 



II. Petrological. 



1. RocTcs in situ: (1) Basalts. — Two specimens from the cliff to 

 north-east of Balfe Point, west of the island, were taken from two 

 bands each about 12 feet thick. The rock is compact, dull brown, 

 mottled with paler brown, splitting with a slabby or platy structure, 

 which is more marked in the upper band. The lath-shaped microliths 

 of plagioclase felspar exhibit a definite orientation, and are imbedded 

 in a small amount of glassy base, with black granules and grains of 

 iron oxide, small crystals of pyroxene, and a few rather larger 

 crystals (usually pyroxene or felspar). Thus it is a not very basic 

 magma basalt, approaching an andesite, and is probably a lava-flow. 



In several of the Perim rocks two apparently ferro-magnesian 

 minerals are found. One is augite, very pale greenish with low 

 polarization tints ; the other is in well-formed crystals, partly or 

 wholly a reddish brown. The central part of these, when it is clear, 

 shows rich polarization colours, has no cleavage, is rather granular, 

 and sometimes contains enclosures. The mineral resembles olivine, 

 but it might be a second pyroxenic constituent, and gradations from 

 undoubted augite sometimes occur. In a few slides it constitutes 

 mici-oporphyritic crystals, but such crystals (few and small) more 

 generally consist of felspar. 



Another rock (from a cliff north-east of the harbour) is dull 

 purplish red, mostly compact, rather coarser-grained than the last 

 described, and without fluxional orientation. It contains one group 

 of larger crystals, consisting of felspar (with glassy inclusions along 

 crystal planes) and of augite. 



(2) Volcanic Tvffs. — In a quarry east of Balfe Point (Fig. 1), 

 some 25 feet deep, the basement rock, a layer about 1 foot thick, is 

 whitish, mostly fine-grained, but with occasional fragments up to 

 ^ inch in diameter, weathers to a crumbling mass, but probably 

 would set into a hai-d stone. Crystalline calcite, sometimes in 



* At one spot, in the north of the Red Sea, a recent accumulation of oolitic grains 

 was described by J. Walther, " Die Korallenriffe der Sinaihalbinsel," 1888. 

 ^ Marked on the Admiralty Chart. 



