ROCK SPECIMENS FROM THE ISLAND OF SOCOTRA. 
285 
rhyolites,” exhibiting on their weathered surfaces a beautifully distinct and rather 
parallel fluidal structure. This is most perfect in the second specimen. Microscopic 
examination shows it to be produced by ferrite-stained bands and lines of opacite 
granules and trichites with clearer interspaces. Small crystals of quartz, felspar, and 
little spherulites occur occasionally. The ground mass seems to be wholly clevitrified. 
The first specimen shows traces of “ flow brecciation” and is more unequally devitrified, 
with but few imperfect spherulites. The last [4214] has irregular spherulites and bands 
with a brush-like arrangement of microliths : it contains a little more free quartz. 
From “the banks of the Kereguiti stream, south of Hadibu,” apparently intrusive in 
the green slaty rocks and the lower part of the overlying limestone, come pale buff or 
grey felsites. [4234] exhibits under the microscope an extremely minute clevitrified 
structure, the slide is slightly clouded with ferrite and contains small scattered granules 
of quartz and felspar and minute specks of ('() hornblende. This rock is not unlike 
some of the most compact varieties of felstone from the Bala group of North Wales. 
From “ the spurs of the Haggier range running towards the sea ” come compact red 
quartz-felsites and rhyolites, generally resembling those already described. [4285] is 
cryptocrystalline, with a rather coraloid or arborescent structure, occasionally spheru- 
litic; it is much stained with ferrite, contains a few scattered grains of quartz, decom¬ 
posed felspar, iron peroxide, and hornblende or tourmaline, and is more probably from a 
dyke than from a flow. 
A pinkish quartz-felsite [4331] “from the interbanded group of rocks underlying 
the conglomeratic base of the limestone, south of Ma-aber,” has a cryptocrystalline 
ground mass of quartz and decomposed ferrite-stained felspar; the former mineral, 
which is very abundant, has at first sight a rounded or polygonal outline, giving the 
rock a superficial resemblance to one of fragmental origin. In this ground mass are 
scattered grains of quartz, crystals of felspar, and (probably) magnetite. In the 
eastern part of the Haggier range near Adona the coarse red pegmatite is cut by dull 
purplish compact felstone dykes. [4399] has been taken from a vein only about §" 
thick. The junction with the granite is beautifully exhibited in the slide, one or two 
small fragments of the latter being included in the former. This exhibits a crypto¬ 
crystalline structure, almost microcrystalline in parts, which, however, is in no respect 
remarkable. In the other [4368], also a junction specimen, the intrusive rock is 
minutely clevitrified, shows slight fluidal structure, and has a general resemblance to 
the rhyolites already described; a small band of crushed granite occurs at the junc¬ 
tion, as shown in the figure (Plate 7, fig. 3). 
The felsites, which are associated with the argillites and calcareous rocks on the 
banks of the Kereguiti stream (Hadibu Plain), being apparently intrusive in them, are 
of a different charaeter; they are compact flinty felsites of a pale buff or grey colour. 
One [4234] under the microscope exhibits an extremely minute cryptocrystalline struc¬ 
ture, the ground mass being irregularly and lightly clouded with ferrite, containing 
