532 Dr. J. W. Gregory — Geology of Socotra, etc. 



In one case the quartz shows a zonal arrangement of the inclusions, 

 a feature unusual in volcanic quartz crystals ; but the identification 

 of the mineral was confirmed hy the use of convei-gent polarized 

 light. The felsite from Adho Dimellus has numerous, somewhat 

 ill-defined spherulites ; in that from Homhil some of the smaller 

 patches of secondary quartz are traversed by faint lines like 

 cleavages, suggesting that the quartz has been formed by the altera- 

 tion of tridymite aggregates. 



The last member of the rhyolite-felsite series is an altered, fine- 

 grained trachyte from the Ghariah Plain. It is similar in character 

 to the trachyte figured by Professor Bonney^ from the Azorah 

 district. 



A dyke of a fairly fresh basalt was found by Dr. Forbes at Thlutid, 

 on the north-west slope of the granite mass of Adho Dimellus. 



The most interesting feature in the trachyte and felsite is that the 

 rocks resemble those of the Aden volcanic series rather than those of 

 the East African volcanic gi"oup. For opportunities of examining 

 some of the Aden rocks I am indebted to Mr. G. T. Prior. 



Hence Socotra appears to be an outlier of the Somali plateau, 

 which has been involved in the movements which formed the Gulf 

 of Aden, and has been the scene of the eruption of volcanic rocks of 

 the Aden series. 



2. Abd-el-Kuri. 



West of Socotra is the small island of Abd-el-Kuri, where a larger 

 rock collection was made by Messrs. Forbes and Grant. It was also 

 visited by Dr. Kossmat, who has described it as composed of amphi- 

 bolites, penetrated by a network of granites and camptonite dykes, 

 and capped by a limestone which he suggests to be of Turonian age. 

 The only previous geological account of the island known to me is 

 by H. J. Carter," who in his memoir on the geology of the south-east 

 coast of Arabia described Abd-el-Kuri as composed of a mass of 

 granite and diorite, capped by a limestone which reaches to the 

 summit at the level of 1600 feet above the sea. 



The collection from Abd-el-Kuri is composed mainly of rocks 

 belonging to the Archean series. They include a series of amphi- 

 bolite schists, a syenitic gneiss, some specimens of which have 

 a considerable development of epidote, and a white gneiss with 

 hornblende aggregations : the last rock is strikingly like one which 

 is common in the Ulu Mountains of Ukarabani in British East Africa. 

 This series of foliated rocks is cut through by pegmatite dykes, 

 which are almost as coarse in grain as those of Somaliland : in one 

 specimen collected the muscovite occurs in crystals from two to 

 three inches in diameter. 



The Archean rocks are covered by limestones which occur at 

 different levels from 40 feet above the sea to the summits of Gebel 



1 Bonney : op. cit., p. 287, pi. vii, fig. 6. 



2 " Geol. Papers on Western India," 1857, p. 620. On p. 621 of the same work 

 Carter describes ' The Brothers,' the islets between Socotra and Abd-el-Kuri, as also 

 formed of granite and diorite capped by limestone. 



