1882.] On a Collection of Rock Specimens from Socotra. 147 



the northern part (if not of the whole island), appears to consist of 

 granitoid gneiss, replaced towards the centre by granite. 



The granitic, felsitic, and rhyolitic rocks mnst occupy a consider- 

 able breadth of the island from north to south, for there are many 

 specimens from districts traversed on the return journey from the 

 eastern promontory, in which Prof. Balfour, after keeping parallel to 

 the southern coast for some miles, struck inland in a north-west direc- 

 tion. Thus measured, there must be an area some ten miles across 

 occupied by these rocks ; and judging from the specimens, one would 

 say that this was one of the chief centres of ejection of rhyolitic lavas ; 

 this is near that part of the island covered by the final A in Sokotra 

 on the map. In crossing back to the north shore along the course of 

 the Haggier river granites, basalts, felsites, and rhyolites, as might be 

 expected, were collected. The conglomerates of felsite and rhyolic 

 pebbles picked up on the Nowkad Plain, approaching the southern 

 coast, show that there must be a large mass of these rocks somewhere 

 on the south flank of the Haggier range. 



The limestone is generally of a yellowish or whitish colour, compact 

 in structure, and often not unlike the dolomites of the Italian Tyrol, 

 in the hand specimen. Microscopic examination shows that it is some- 

 times partially dolomitized. It contains numerous foraminifera 

 amphistegina, globigerina, textularia, rotalina, &c. The first of these 

 shews that it is probably of Middle Tertiary age, and thus rather 

 later than the limestone of the Sinai Peninsula. 



The author's investigations lead then to the following conclusions : 

 That the oldest rocks in Socotra are gneisses, hornblendic, and grani- 

 toid, belonging, like those of the north-west of Scotland, of North- 

 east America, &c, to the earliest Archsean age. That these, as at 

 Sinai, are broken through by granites, some of which resemble much 

 those of Serbal and Jebel Musa, and that these are cut by later 

 granites, felsites, and greenstones, together with basalts, the last pro- 

 bably of rather recent date. On the southern flank of the Haggier 

 range, there must have been a rather extensive volcanic disturbance, 

 from which rhyolitic lavas, often showing marked fluidal structure 

 and scoria were ejected. The date of these eruptions cannot be fixed, 

 but it was prior to the deposition of the limestone, and may be much 

 older, except locally, where there is a little sandstone possibly repre- 

 senting the Nubian sandstone (Carboniferous) of Sinai, and the 

 argillite. What is now Socotra, would appear to have been a land sur- 

 face from very early times, until the submergence in the Miocene 

 period, when the great masses of limestone were deposited. It is, 

 however, quite possible that the peaks of the Haggier range may have 

 remained above water even during that time. Since its elevation, 

 great denudation has doubtless taken place, including the definition 

 of the island, and the sculpturing of the valleys in the limestone 



l2 



