Ümiir•tâgh. Cliffs near C. 219 (or).
Riebeckile Granophyre. A ' joint ' fragment of a compact blue-grey rock.
Under the microscope it presents a colourless transparent ground mass which is shot through with numerous dark bristle-like prisms of a ferro-magnesian mineral. Between crossed nicols it is resolved into radiate micrographic inter-growths of quartz and felspar ; probably a soda orthoclase. Near the centre of these intergrowths the two minerals are in the form of fine threads, sometimes as fine as in a spherulitic growth, but they enlarge as they radiate outwards, and the quartz is continued beyond the felspar to end in wedge-shaped sections or to form a continuous border to the whole growth.
The ferro-magnesian minerals are of two kinds, readily distinguished by their difference in colour, one being of a blue tint, the other almost colourless, but with a slight tinge of green. The blue mineral is evidently riebeckite ; it extinguishes at a small angle and is highly pleochroic : X, blue; Y, blue ; Z, faint yellowish green. The faint green mineral agrees in all its characters with aegerine.
Bai. C. 219. Jasper and Felsfile.
A worn fragment of red jasper and dark grey felsite.
The jasper consists of quartz crowded with ball-like growths and minute particles of ferric oxide. It has been brecciated, and the widely open intervals between the resulting fragments filled with quartz, epidote, and small crystals of magnetite. The consolidated rock thus produced was again fractured and recemented with quartz, which forms very thin
ROCK AND SAND [Appendix O
veins running in various directions and of various length, some extending across the whole section.
The grey rock, which is sharply defined from the jasper, represents the original igneous material which was transformed into that substance, and though not jasperized itself it is highly altered. It is not homogeneous. The greater part is a uniform microcrystalline ground mass; the remainder is similar, but contains phenocrysts of a felspar which, though thoroughly sericitized or converted into obvious crystals of muscovite and sometimes of epidote, still, in some cases, retains traces of albitic twinning.
The whole of the grey rock, which we may doubtfully term a felsite, is traversed by thin veins, some of which extend through it and the jasper. They are usually filled with quartz, but some, more irregular in form and less straight and continuous than the rest, with calcite.
Bai. Orpiment Limestone.
This is a very fine-grained limestone composed of extremely minute granules of calcite and traversed by many fine veins in various directions ; in and about these the calcite forms a mosaic. Evidently the limestone was brecciated after consolidation, and in some of the resulting fissures orpiment was deposited ; this sometimes lines the sides of the veins and gives off short processes into the surrounding matrix ; if the vein is very narrow, it may be filled with a string of orpiment crystals unaccompanied by calcite. In places the orpiment forms large ' bunches ' and thus seems to have replaced the calcite.
Kuruk-tagh. C. ccxli. Malachite.
A fragment of vein-stone consisting of a coarse mosaic of calcite invaded by quartz and malachite. The calcite is dusty with ferric hydrate and contains patches of quartz and malachite, but it also appears to have grown upon already deposited malachite. The malachite is for the most part granular, the granules being apparently minute crystals, but in some places it occurs in stellate, spherulite-like growths of long radiating prisms. These are pleochroic, with X colourless ; Y, bluish-green.
Kuruk-tâgh. C. ccxli (02). Rhyolite.
A worn fragment of light red rhyolite, sp. gr. 2.59. The ground mass consists of minute crystals of quartz and felspar, through which are scattered micrographic growths of quartz and felspar with broken and corroded phenocrysts of quartz.
The micrographic growths rarely assume a spherulitic form ; most commonly their form has been determined by the felspar, so that they build up micrographic crystals of this mineral. Sometimes, however, the growth escapes from this influence and is bounded by a curved outline.
Ferro-magnesian minerals are absent, magnetite is very rare, but there are abundant granular patches of ferric hydrate, and to these the rock owes its colour.
45 a. C. ccxli. Quartz.
Konche.darya. C. ccxlix (o4). Mica Schist.
Kuruk-tâgh, P`o-ch`êng-tzû? (os).
About one half of this rock is a coarse mosaic of calcite which fills what were originally large winding cavities in the unconsolidated lava. The other half was itself highly vesicular, the vesicles occupying more space than its substance. They are mostly circular or elliptical in section, sometimes confluent, and occasionally drawn out into long parallel tubes as in pumice. They are now filled sometimes with quartz, sometimes with felspar, at others with calcite or chlorite, or with more than one of these minerals. Quartz, when it occurs with chlorite, usually lines the wall of the vesicle, leaving the central space to the chlorite ; when it occurs with calcite, it may fill one half of the vesicle and the calcite the other. The vesicles are bounded by a definite wall consisting of opaque white material, which, however, under high magnification is resolved into a granular aggregate of colourless transparent substance which has a high refractive index.
The substance of the lava consists of irregular intergrowths of quartz and felspar. Some glassy clear oligoclase-albite is often present, but crystals of orthoclase have not been observed.