282 
PROFESSOR T. G. BONNEY ON A COLLECTION OF 
origin, bordering augite crystals, but one or two grains of darker colour may possibly 
be original constituents. The latter specimen then comes near to the true dolerites, 
the former to the Labrador-diorites. 
From boulders in the bed of a stream-course near Fernaighey (no rock occurring in 
situ), come some specimens, of which [4499] may be taken as a type. The ground mass 
consists of a mass of small crystals of a white felspar and a black mineral, in which 
are scattered larger felspar crystals, sometimes 2 inches long. Under the microscope 
the rock is seen to consist of a rather decomposed labradorite, of a dull green, dirty - 
looking mineral associated with and occasionally replaced by opacite, probably a decom¬ 
posed augite or diallage, and of a fibrous or scaly green mineral, giving bright tints with 
crossed nicols, also associated with grains of opacite. These aggregates have probably 
replaced olivine. There are also some fair sized crystals,of apatite. The rock then 
has been an olivine-dolerite, but would now be more properly classed with the 
diabases. 
Granites. 
For the reasons given above I am unable to decide whether some of the specimens 
brought by Professor Balfour are true granites or only exceptional examples of the 
granitoid gneisses or granitoiditesA At the same time we need not hesitate to admit 
the existence of a considerable amount of this rock, especially in the more central part 
of the island, as for instance in the Haggier range. The granites of Socotra, as a rule, 
consist mainly of quartz and felspar, and so belong, as we have said, to the variety 
pegmatite. The latter mineral usually predominates and is commonly of a reddish 
colour. They are frequently much decomposed and not seldom show some indications 
of a graphic structure. It will suffice to select two or three from a large series as 
examples. [4249], “from the high cliff on the northern face of the Haggier range, one 
of the specimens typical of the rock forming the highest peaks,” is a good example 
of the coarse red and white pegmatite. Microscopically it consists of a decomposed 
felspar, most, if not all, being orthoclase, quartz with numerous microlithic enclosures 
and minute fluid cavities, and a little of a dark green strongly dichroic mineral. Some 
of this is hornblende, but part of it much resembles tourmaline, to which mineral some 
acicular microliths, in one case in a tufted group, almost certainly belong. A. second 
[4241] “from the slopes above Hadibu,” has among it felspar, microcline, and a fair 
quantity of plagioclase (? oligoclase), grains of magnetite and a green chloritic mineral 
associated wfith clots of opacite, evidently replacing a magnesia-mica, associated with 
which is a little of a colourless mineral, possibly apatite. [4264], from the same region, 
is a “ vein-granite, which cuts the coarser variety.” It is a very finely" crystalline 
quartz-felspar rock of a warm grey colour, containing some dark crystals about 0 ‘25" 
long. With the microscope it is seen to consist of intercrystallisecl quartz and felspar, 
the latter often exhibiting close twinning and sometimes being probably" microcline. 
* Quart. Jom\, Geo. Soc., vol. xxxv., p. 322. 
