ROCK SPECIMENS FROM THE ISLAND OF SOCOTRA. 
283 
I 
The crystals occasionally exhibit rectilinear boundaries, but often interlace one with 
i another and with the quartz in a very complicated way, as is not seldom the case in 
vein-granites. The slide contains some brown mica and tourmaline, the latter certainly 
a pseudomorph after the former. Two crystals retain a cleavage resembling hornblende, 
but behave optically as a uniaxial mineral, and when properly placed absorb the 
polarised beam far more completely than is usual with hornblende—a curious case of 
, pseudomorphism, which, however, is not without parallels (Plate 7, fig. 2). [4205], 
an erratic near Kadhab village, doubtless from the western part of the Haggier range, 
is a red felspar-granite rather similar to the last, with a little iron-mica. [4207], a 
rather similar rock from the same region, exhibits in part a distinct micrographic 
structure; the decomposed felspar crystals are intimately interbanded with quartz, 
possibly of secondary origin; these two are most likely vein-granites. [4439], near 
Hesainho, in a region cut by felstone dykes, is a fine-grained pink and white granite 
with green specks, which under the microscope shows intercrystallised quartz and 
decomposed felspar (orthoclase and (?) oligoclase), with an approach to a graphic 
structure, besides some altered biotite—the rock is cut by a dyke of gabbro. Between 
these two rocks is a thin zone composed of the granite and the gabbro crushed, and to 
sofne extent mingled, though the materials of the former predominate. The constituents 
have been subsequently recemented, probably by deposition of quartz; this crushing w 7 as 
doubtless subsequent to the solidification of the newer rock. From the north-west of 
the Girgha range comes a series of specimens varying from coarse to moderately fine 
crystalline, and consisting of quartz and felspar with small quantities of hornblende 
and black mica, having a general resemblance to those already described from the 
Haggier range. Microscopic examination of one [4335] shows that the felspar is ortho¬ 
clase, and perhaps microcline, with oligoclase. Small quantities of a chloritic mineral 
and a few microliths, possibly of tourmaline, are also present. The structure of the 
rock is rather abnormal for a granite, having some resemblance to the granitoid gneisses, 
but as there has been some local crushing and recementation, it may, notwithstanding, 
be a true granite. 
Felstones and rhyolites. 
Under this head I have retained a large number of rocks, which in some cases it 
would have been easy, but in others impossible, to subdivide. Petrologists are at 
present hardly in a position to agree upon precise definitions for the names of certain 
of the more acid igneous rocks, or upon the classificatory value of some of their 
minute structures. For example, a felstone must have a ground mass which is either 
microcrystalline or cryptocrystalline. But at present we cannot say whether or not 
these two structures correspond with different conditions in the past history of the rock, 
and so are of specific rather than of varietal value. Again, the latter structure, in 
some cases, appears to have been produced during the solidification of the mass, in 
others long afterwards. These, at present, it is often impossible to distinguish. Again, 
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