OCT 1 1900 



THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. VI. 



No. XII.— DECEMBER, 1899. 



I. — A Note on the Geology of Socotra and Abd-el-Kuki, 



By J. W. Gregory, D.Sc, F.G.S., of the British Museum (Xatural History). 



rpHE first account of the geology of Socotra we owe to Lieutenant 

 JL J. R. Wellstecl,^ who compiled the Admiralty chart and map, 

 and in 1835 described the island in a detailed memoir, in which he 

 showed that it consists of a mass of granite capped by limestones. 

 Nothing material was added to this description until 18So, when 

 Professor Bonney published his account- of the extensive rock 

 collection made by Professor J, B. Balfour during a six weeks' visit 

 to the island in February and March, 1879. Professor Bonney 's 

 study of the rock specimens enables him to prove that Socotra 

 consists of a block of Archean rocks covered in places by massive 

 limestones, which, on the evidence of their foraminifera, Professor 

 T. E. Jones suggested were probably of Miocene age. It was 

 further shown that both the Archean series and the Oainozoic lime- 

 stones are cut through by dykes of trachyte and basalt, associated 

 with a series of rhyolitic lavas. Professor Balfour's expedition not 

 only proved that the foundations of Socotra are built of old materials, 

 but that the island is of considerable antiquity, as is indicated by its 

 rich endemic flora.^ 



In 1880 Socotra was visited by Emil Eiebeck, who collected some 

 rock specimens containing the mineral which was described in 1888 

 by Sauer under the name of Eiebeckite.^ Further details regarding 

 the Archean and volcanic rocks were given in the same year in 

 a paper by Miss Eaisin.^ 



1 J. R. "Wellsted, " Memoir on the Island of Socotra" : Journ. Roy. Geoo-r. See 

 vol. V (1835), pp. 129-229, and map. ° '' 



2 T. G. Bomiey, " On a Collection of Rock Specimens from the Island of Socotra "- 

 Phil. Trans., vol. 174 (1883), pp. 273-294, pis. vi and vii. Some preliminary 

 remarks on the geology of the island were included in Professor Balfom-'s report, 

 " On the Island of Socotra " : Rep. Brit. Assoc, 1881, p. 486. 



3 According to Balfour {" Botany of Socotra" : Trans. Roy. Soc. Edinb., vol. xxxi, 

 1888, p. Ixxv) the island has been a land area since the Permo-Carboniferous, and 

 " an island certainly from Tertiary times." 



* Sauer, " Ueber Riebeckit, ein neues Glied der Hornbleudegruppe " : Zeit dent 

 geol. Ges., vol. xl (1888), pp. 138-146. 



5 C. A. Raisin, " Rock Specimens from Socotra " : Geol. Mag., Dec. Ill, Vol Y 

 (1888), pp. 504-7. 



DECADE IV. VOL. VI. — NO. XII. 34 



