34 J. Parkinson — The Darjiling Gneiss. 



about 9 inches, the thickness 2-5 inches. The specimen was rather 

 darker in colour than that above described, with a siliceous aspect, 

 and faintly mottled with red garnets (Fig, 2). The quartz grains 

 are subangular in outline, occasionally exhibiting crush shadows. 

 They contain very many small crystals of a colourless mineral in 

 the form of prisms having lath - shaped sections, which I am 

 unable to name. The groups of garnet grains show a distinct 

 attempt at a general linear arrangement, and a similar elongation 

 occurs in all the minerals. The slide contains sphene and at least 

 two other minerals which are noteworthy. I am indebted to 

 Canon T. G. Bonney for suggestions concerning these. First, 

 a considerable quantity of an opaque mineral characterized by its 

 elongated shape (commonly about -0038 x "00055 inches). The 

 outlines are rather jagged and uneven, the colour by incident light 

 is grey, and the lustre metallic. It is associated with a little 

 specular iron, which is also met with elsewhere in the slide. In 

 order to test the streak I powdered a small quantity of the rock, 

 and by means of a lens isolated a few of the black flakes and 

 pressed them with a knife blade upon a sheet of notepaper. 

 They yielded readily to the pressure and left a dull grey mark. 

 I concluded, accordingly, that the mineral is graphite. It occurs, 

 though less plentifully, in slides of other nodular eyes. The second 

 mineral usually forms small and more or less rounded grains. An 

 idiomorphic outline is sometimes indicated by the occurrence of 

 stumpy rectangles, (extinction takes place parallel to the sides) 

 with unequally truncated corners. The refractive index is higher 

 than that of apatite, to which we see a resemblance in double 

 refraction, slightly bluish colour, roughened aspect, and absence 

 of cleavage. A grain occasionally appears isotropic. I incline to 

 refer the mineral, but with hesitation, to idocrase. 



Another specimen from the same locality, as seen in a thin 

 section, consists of a network of large quartz grains and very 

 irregular plagioolase, exhibiting as before much micropegmatitic 

 quartz. Garnets, some dozen grains of sphene, and a number of 

 mica flakes make up the rest of the slide. 



On the western side of the Darjiling ridge, near St. Joseph's 

 College, scattered sections of the gneiss are found sufficient to 

 enable its characters to be studied. The rock is frequently gnarled 

 with irregular bands ; once I saw such a one, rich in quartz and 

 felspar, diminish in thickness in 3 feet from 9 inches to 2 5 inches. 

 Close by, a broad band at least a foot across was streaked and veined 

 irregularly by darker, i.e. by more micaceous, material. Parallel to 

 this were others shaped like very elongated lenticles. Again, the 

 gneiss on Birch Hill Eoad was occasionally beautifully puckered 

 in successive V's ; and some infiltration veins had been formed 

 subsequently to the folding. The gneiss in situ about three- 

 quarters of a mile from Darjiling is a finely foliated, slabby rock 

 of medium grain, containing a considerable quantity of mica, 

 A thin section discloses the presence of sillimanite, white and 

 brown micas, and a few irregular pinkish garnets about "04 inch 



