Appendix O] SPECIMENS OF ROCK AND SAND 1065
ochreous yellow ; Y and Z, deep sage green to black ; most commonly black. It includes some rounded crystals of zircons. Pleochroic haloes were looked for, but none were found.
A few rare crystals of other minerals are present, a little apatite and zoisite. One elongated crystal of zoisite is particularly interesting, since it has been broken across in several places in the direction of pressure, i. e. at right angles to the general parallelism, and one of the fractures has been enlarged so as to produce a considerable interval which has been filled by a growth of muscovite.
35. Pei•shan, C. 212 (o6). Quartz Schist.
This is a small fragment of a compact rock and in the hand specimen shows no signs of schistosity ; under the microscope, however, it is seen to be definitely foliated. It consists chiefly of a quartz mosaic, the elements of which show undulose extinction and are elongated in one direction, as also are the other constituents of the rock. These consist of muscovite and biotite, which are closely associated ; indeed the biotite appears to pass into muscovite. The biotite is
strongly pleochroic : X, faint brown; Y and Z, completely ubsorptive, i. e. black, even in thin films. Felspars are absent, but a rounded and broken grain of epidote is sometimes to be seen.
36. Nan•ch`tian, Pei-shan (o4). Nineteen miles NW.
of Nan-eh`iian (9. ix. 1914). Dolerite.
A fragment of a dark grey rock ; sp. gr. 2.95.
A thin slice presents plank-like and occasionally tabular sections of Labrador felspar, often ophitic with augite. In some cases the two minerals form roughly radiate inter-growths, a rude attempt at a spherulite on a grand scale.
The labradorite is very .much altered (sericitized?), and loaded with the products of alteration, especially in the middle, the margins alone remaining clear. There is a singular rarity of albitic twinning. It often shows undulose extinction. The augite is colourless, where fresh, but gives rise by alteration to green chlorite.
Ilmenite occurs in abundance, sometimes as skeleton crystals, but is not bordered by leucoxene.
37. Ümür•tâgh. Cliffs in gorge sixteen miles NW. of
C. 218 (or). Quartz Monzonile.
Hypidimorphic granular rock composed of orthoclase, microperthite, albite and quartz, pyroxene, amphibole, biotite, apatite, titaniferous magnetite, sphene, and rare zircons.
The apatite and no doubt the zircons were the first to crystallize out, then followed the titaniferous magnetite, which includes apatite and is corroded by quartz. This mineral is bordered by colourless transparent sphene. The pyroxenea colourless diopside with diallage structure—next made its appearance ; it rarely or never presents crystal outlines, owing to the transformation of its outer part into a green hornblende, which presents the following pleochroism : X, colourless or faint yellowish green ; Y, deep green, occasionally bluish green ; Z, green or bluish green ; and extinguishes at a high angle, as much as 25° : it closely resembles Hastingsite.
The biotite, at least in part, is later than the hornblende ; in one instance it includes a corroded crystal of hornblende which itself encloses a core of augite. It occurs in rather large crystals, brown when fresh (green when altered), and strongly pleochroic : X, clear yellow ; Y and Z, deep brown. A good deal of apatite, sometimes in unusually large crystals, is included in the biotite, but no zircons. The only zircon observed was in quartz.
The felspars followed the biotite, and the orthoclase the albite which occurs in idiomorphic hypidiomorphic crystals, sometimes corroded and embedded either in orthoclase or quartz. The orthoclase and microperthite form large hypidiomorphic crystals.
The quartz, which was the last to consolidate, is interstitial ; it has acted as a corrosive solvent on most of the other minerals.
Some secondary sphene and calcite are present in the rock.
Ümür•tagh. From big rock wall, six miles NW. of
C. 218 (02). Pegmatitic Granite.
Coarse pegmatitic granite with red felspar, chiefly orthoclase of sp. gr. 2.55. Sp. gr. of rock, 2.61.
Ümiir•tagh. Thirteen miles NW. of C. z r8 (os).
Granophyre. A fragment of a brownish-grey fine-grained rock with
numerous internal close fractures.
It consists of a micrographic growth of quartz and orthoclase ; primary ferro-magnesian minerals are absent, but are represented by some rare sporadic crystals of epidote.
The micrographic or micropegmatitic material has been deformed by pressure and in many cases sheared into phacoids which are sometimes well defined by thin lines of an opaque white substance which appears to be altered epidote.
The thread-like rays of the micrographic radioles have been bent out of their original course, quartz grains have been crushed along the margin and reconstituted : they show undulose extinction, and this is represented in the felspar crystals by irregular bands crossing a section, which extinguish at a slightly different angle from the rest of the crystal.
A general parallelism of structure has been impressed upon the rock.
Ümiir-tâgh. From a decayed ridge, nine miles NW.
of C. 218 (04). Arcalite (Aphte).
A salmon-red rock composed almost entirely of quartz and felspar. The quartz is dusty with vapour cavities and undetermined particles. The felspar is orthoclase, microcline, and albite in hypidiomorphic crystals.
Some altered biotite, corroded by quartz and associated with sphene, is also present as well as a little magnetite.