ROCK SPECIMEN'S FROM THE ISLAND OF SOCOTRA. 
289 
rocks does not disappear under the microscope, and I think we have in this group 
of specimens evidence of volcanoes of very considerable geologic antiquity. 
The conglomerates of rhyolite and felsite, which occur as boulders on the Nogad 
plain (p. 278), though their materials have been derived from the same group of igneous 
rocks, are probably far more modern than the last. These I have not deemed it 
necessary to examine microscopically. Breccias and conglomerates of various kinds 
are not infrequent in Socotra, but generally present no special interest. Some occur just 
at the base of the limestone series, others may be quite modern; subserial or stream 
detritus, cemented by tufaceous deposit. 
Argillites. 
This group of rocks, found underlying the limestone near Hadibu Bay (p. 27G), is 
one upon the exact nature of which it is difficult to decide. I am not able to satisfy 
myself, from the specimens which I have received, whether there is a true cleavage or 
not; the rock has a somewhat porcellaneous aspect, as if it were at least of palseozoic 
age, but a.s there appear to be intrusive rocks in the neighbourhood, one hesitates 
without further evidence to assign it to a remote geologic period. As, however, there 
is at the base of the limestone a breccia of fragments, seemingly in the same mineral 
condition, and as there seems to be an unconformity here, the interval between the two 
groups of rocks is probably considerable. Shaly bands are, indeed, said to occur in the 
lower part of the limestone, but it is possible that these may be formed of detrital 
material from the older group. Microscopic examination does not throw much light 
on the history of the latter. The specimens are composed of earthy material and 
minute, rather angular, fragments of a clear mineral (quartz, with possibly some 
felspar), and a number of exceedingly minute, doubly-refracting, rather highly-tinted 
granules; these are probably in some cases a variety of hornblende, in others epidote. 
Limestone and dolomite. 
Several specimens of the massive limestones which form so much of the upland 
district in Socotra are among Professor Balfour’s collection. Generally they are 
compact in texture, with a clean subconchoidal fracture, buff, whitish, or sometimes 
pinkish in colour ; in aspect often remarkably like the ordinary dolomitic rock of the 
Italian Tyrol. The weathered surfaces are sometimes curiously rough and cavernous. 
I have examined microscopically a few specimens from different parts of the island. 
The first [4168] is one of a series collected in the district extending from the head of 
the Gollonsir Valley towards the Kadhab Plain. These are compact in structure with 
a subconchoidal fracture, buff coloured and pinkish, the former being more crystalline. 
The specimen examined is a very pretty pinkish rock that would take a good polish. 
It contains numerous organisms, both tolerably perfect and fragmentary, among which 
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