273 
PROFESSOR T. G. BONNET ON A COLLECTION OF 
to resemble the neck or “core” of a volcano. Near the opening of the gorge are 
doleritic rocks, which extend for some distance, and are, at any rate in some cases, 
intrusive in the granitic series which is sometimes fine-grained and is more 
hornblendic than usual. The red felsite also occurs. 
After returning to Hadibu Professor Balfour crossed the Haggier range, and 
travelling in a general southerly direction reached the Nogad plain about the middle 
of the south coast of Socotra, But few specimens were collected during this part of 
the expedition, and the only two of interest were a red rhyolite, near Feraighey, and a 
rather peculiar dolerite, which will be more minutely described under their proper 
heads. On the plain were loose blocks of a hard conglomerate containing well- 
rounded pebbles (up to about 1 inch or longer in diameter) of compact red quartz- 
felsite and purplish rhyolite with well marked fluidal structure embedded in a silicious 
paste. Besides these, blocks were found of white cherty fragments, also in a silicious 
matrix, which contains grains of the above igneous rocks. After leaving the Nogad 
plain Professor Balfour again crossed the limestone plateau to the north coast near 
Gubbet Kadhab, whence he returned to Hadibu. 
I now proceed to describe the more conspicuous and interesting varieties of rocks in 
Professor Balfour’s collection, taking the more crystalline and most ancient series 
first in order. 
Gneissic rocks. 
The existence of a series of highly crystalline metamorphic rocks in the Island of 
Socotra is indubitable, notwithstanding the difficulty already mentioned, of deciding 
without further examination whether certain of the sjoecimens are to be referred to 
the granitoid gneisses or the granites. This series forms the floor on which rest the 
great limestone plateau and some other sedimentary deposits, and is well exposed in 
the western and to some extent in the eastern part of the island, being replaced in the 
central and most elevated region by true granites. The following are the principal 
varieties : (a) gneisses, not conspicuously foliated, consisting of quartz, felspar (gene¬ 
rally of pinkish colour) and mica, black and white, and possibly hornblende. As a 
type of these a specimen from a hill near the opening of Gollonsir Valley was 
examined microscopically. The slide exhibits quartz, felspar, little elongated clusters 
of green hornblende with some epidote, opacite, ferrite, and a few scales of white mica. 
The felspar is chiefly microdine, a variety which is remarkably abundant in the 
Hebridean gneisses and in others of great antiquity. There is also a plagioclase, 
probably albitey and some of the felspar crystals contain groups of small colourless 
microliths with a parallel arrangement. This structure I have also observed in some 
of the old gneisses from Greenland. The quartz contains small cavities (rarely with 
* In attempting to determine the species of the felspar I have made use of the optical tests given by 
Professors Fotjque and Levy (among others) in their magnificent work “Roches Eruptives Francaises.” 
I cannot, however, say that I feel great confidence in the results of the method. 
