32 J. Parkinson — The BarjUing Gneiss. 



are only just discernible to the naked eye, and the presence of mica 

 gives it a uniform grey tint. Part of the specimen is a much 

 coarser compound of quartz and rather decomposed felspar, with 

 but a few flakes of mica, and containing a band, '1 to '15 inch 

 thick, consisting almost entirely of the last-named mineral. The 

 thin section shows the part of finer grain to consist of the following 

 constituents, the whole rather crushed : brown mica which is 

 plentiful, some white mica in part secondary, zircon fairly common, 

 garnet rare, quartz the most abundant mineral, orthoclase and 

 plagioclase. The felspars are commoner in some parts of the slide 

 than in others. The plagioclase, which appears to be albite or 

 oligoclase, is often rather unusually translucent. The quartz forms 

 large irregular grains, and occasionally occurs as quartz vermicule. 

 The orthoclase frequently contains a microperthite intergrowth. 

 Sillimanite is found in another specimen from the same locality. 



Fig. 1.- — Usual type of the Darjiling gneiss, x 20. An irregular crystal of garnet 

 appears on the left side towards the top of the stage. Near it are crystals of 

 ■white mica {m.w.). The centre of the field is occupied by brown mica {m.b.) ; 

 basal sections are cross-hatched. Sillimanite {si.) is seen on the leftside. The 

 rest of the section is composed of orthoclase {of.), quartz {q.), and a single grain 

 of sphene (s.). 



Amongst a pile of trimmed blocks by the roadside, I found one 

 with a hard nodule-like ' eye ' some six inches across, surrounded 

 by the ordinary gneiss. The whole of the periphery was not seen, 

 but the mica folia of the gneiss appeared to bend round it. Fracture, 

 texture, and composition distinguish this inclusion from the ordinary 

 rock. It is hard and compact with no conspicuous foliation, and 

 breaks with almost equal ease in any direction. It has a rather 

 dirty grey appearance from the presence of black specks of mica, 

 •007 inch and under in length, and it is faintly mottled by 

 pinkish patches consisting of clusters of garnet grains, each group 

 being roughly -1 inch across. Under the microscope two principal 

 constituents appear in addition to the garnet, viz. quartz and felspar. 

 The latter forms an allotriomorphic network, indented and embayed 



