INTRODUCTORY CHAPTER. XXlii 



containing little besides quartz and felspar which form the central nucleus of 

 fantastic peaks, the highest part of the island. Cutting through both the fore- 

 mentioned series we have other granitic rocks, such as minette, felsite, rhyolite, 

 and also basalt and diorites, in many places forming large dykes, and in others 

 extensive lava-flows. The centres of ejection of these rocks we were not 

 able to determine, and possibly many of them, as in the case of the tertiary 

 volcanic rocks of east Hindostan, have been discharged, not from cones, but as 

 outflows from fissures. Towards the south-east end of the island they are found 

 in greatest abundance, and there they exhibit a very fluidal character. The date 

 of the eruption of these rocks was certainly pre-miocene. An indurated shale 

 (argillite) is found in some localities, notably on Hadibu plain, and with it a 

 little sandstone of uncertain date, but probably representing the well-known 

 Nubian sandstone of carboniferous age. Over all comes a capping of limestone, 

 forming plateaux over wide areas, rising in abrupt cliffs two or three hundred 

 feet high. It is generally of a yellowish or whitish colour, compact, and some- 

 times slightly dolomitised. It contains numerous Foraminifera, which prove 

 it to be probably of middle tertiary age, or rather later than that of Sinai and 

 the Arabian shores of the Red Sea. The surface of the limestone over 

 extensive districts is rotted and broken into a jagged surface, over which 

 progression is by no means easy, whilst at other spots it forms broad smooth 

 slabs. Subsequent to the laying down of the limestone there occurred further 

 volcanic disturbance, and the limestone is cut through by dykes of basalt and 

 compact trachyte of late tertiary age. 



"We have in Socotra, it seems to me," writes Prof. Bonney, from whose 

 account of our collections I have derived the information just given, " evidence of 

 rocks of an immense, and a land surface of a very great, antiquity. Excepting this 

 argillite of uncertain age and limited extent, and perhaps some sandstone (also 

 local), there is no evidence in the specimens before me to show that this island 

 was submerged during any part of the palaeozoic or mesozoic period. During 

 the kainozoic it undoubtedly shared in the downward movement which affected 

 so large a portion of the globe in and about the north African and Mid- 

 Asiatic districts ; but I should infer that the invasion of the sea commenced 

 much earlier in the Sinaitic peninsula, and think it possible that the topmost 

 peaks of the Haghier mountains were at no time wholly submerged. As it 

 again rose from the waves, perhaps being for a while connected with the 

 African continent, the meteoric forces resumed their work of sculpture, and the 

 waves began their work of insulation. Since then the fauna and flora have 

 undergone their own modifications, but in the Haghier hills we have probably 

 a fragment of a continental area of great antiquity, and of a land surface which 

 may have been an ' ark of refuge ' to a terrestrial fauna and flora from one of 

 the very earliest periods of this world's history." 



