J)r. J. W. Gregory — Geology of Socotm, etc. 531 



•than the rock which forms the main mass of the East African 

 Archean plateau ; but a specimen from Thlutid, at the height of 

 "900 feet, corresponds with the typical gneiss of the mainland. With 

 the granitoid gneiss there is a coarse pink granite which forms the 

 -summit of Gebel Haggler, the highest peak in Socotra; it occurs also 

 ■at Adho Dimellus, and at the height of 1500 feet at Fadehen. 



The second group of the Archean series consists of dark-green 

 amphibolite schists, of the type that forms such a conspicuous 

 feature in the East African gneiss series. This rock was collected 

 'by Dr. Forbes in the Hadibo Plain near Tamrida. 



The Archean rocks of Socotra therefore resemble those of the East 

 African Nyika series, and the island is simply an outlier of the East 

 African plateau. 



Overlying the gneisses is a compact, massive limestone which has 

 been sometimes rendered coarsely crystalline, probably by contact 

 nietamorphism. This limestone forms the extensive plateau at the 

 eastern end of the island, and according to Dr. Forbes sections cut 

 by the Goahal river show that the limestone there rests directly on 

 the Archean series. South of the Goahal river the limestone forms 

 a plateau, the southern end of which is continued westward round 

 the southern flanks of the central highlands. From the Hamadari 

 plateau a belt of limestone extends westward to beyond Tamrida ; the 

 rivers have cut through it, showing that it rests on the Archean 

 series. 



The collection gives no evidence as to the age of the limestones, 

 but fortunately Herr Kossmat has obtained from it a number of 

 mollusca and echinids, which enabled him to determine it as Eocene. 

 Kossmat also records an underlying Cenomanian sandstone with 

 ■Janira quinqiiecostata. 



The last group of rocks represented in the collection is a series of 

 recent volcanic rocks, of types already described from Socotra by 

 Professor Bonney and Miss Eaisin. 



The volcanic rocks include rhyolites, quartz-felsites, and basalt. 

 Most of the specimens are dyke rocks, but one lava is included in 

 the collection. It is a rhy elite with well-marked fluxion structure 

 and containing many angular fragments ; it was collected on the 

 edge of the plateau, south of the Goahal river. A similar rock has 

 been described and figured by Professor Bonney ^ from Azorah, and 

 according to Dr. Forbes the rounded hill in the Ghariah Plain near 

 Kamakanu consists of the same rhyolite. 



Quartz-felsites were collected from dykes intrusive in the Archean 

 series at Homhil and Adho Dimellus. The fel sites at both localities 

 are intensely altered by decomposition ; the abundant iron-ores have 

 been altered to leucoxene, and the matrix is stained bright red. The 

 only unaltered minerals are the large phenocrysts of quartz ; and 

 they are deeply corroded, as illustrated by Velain ^ from the quartz- 

 trachytes of Aden, and by Miss Kaisin^ in a quartz-felsite from Socotra. 



^ Bonney: op. cit., p. 287, pi. vii, fig. 5. 



2 Ch. Velain: " Descript. geol. presqu'ile d'Aden," 1878, p. 18, fig. 2. 



2 Raisin: op. cit., p. 505, fig. 1. 



