J. Parkinson — The Darjiling Gneiss. 



33 



by large quartz crystals and lioneycombed by a pegmatitic inter- 

 growth of the same mineral. A large grain adjacent to a felspar 

 does not as a rule send offshoots into it. Only small groups of the 

 quartz granules polarize together, though we find exceptions. Nearly 

 one-half of the felspar shows no twinning, and the remainder, 

 judging by the extinction measured parallel to the trace of the 

 plane of twinning, is much more basic than might be supposed, 

 symmetrical angles ranging from 35° to 40°. The larger quartz 

 grains exhibit rows of inclusions parallel the one to the other. 

 The garnets are a pale claret-red, subangular in outline, and 

 occasionally making an attempt at idiomorphism, and rather cleaner 

 than those of the gneiss. Grains of zircon and sphene are not 

 uncommon. Flakes of brown mica are scattered sparingly and 

 rather irregularly across the slide. 



Fig. 2.— Nodule -like inclusion, x 20. Near the centre of the stage, or slightly 

 below it, the following letters are jjlaced to indicate the various constituents : 

 gt. garnet, p. plagioclase (faintly dotted), gr. graphite, q. quartz (colourless), 

 i. ? idocrase, s. sphene. The small colourless microlites mentioned in the text 

 are also indicated. 



Passing away from the centre of this 'eye' towards the 

 surrounding gneiss, we find that this hard and compact rock 

 graduates into one of slightly granular texture, rather whiter and 

 containing larger flakes of brown mica and less clear garnet spots. 

 A thin section cut near the junction shows the latter mineral 

 associated with some good-sized crystals of sphene.^ 



On the western side of the ridge the same general characters are 

 repeated. Thus, on the road from Darjiling to Ghoom, a common 

 type of the gneiss contains numerous streaks or elongated lenticles 

 of quartz and felspar, arranged usually in an undulating manner and 

 occasionally retort-shaped in section. The rock weathers to a grey 

 tint and contains characteristically a rather large quantity of mica. 

 A hard lenticle or eye, resembling that noticed as occurring in the 

 gneiss of the Eungeet Eoad, was found in this rock. The length was 

 ^ Occasionally '02 inch long. The slide contains a little augite. 



DECADE IV. — VOL. IX. — XO. I. 3 



