J. Parkinson — The DarjtUng Gneiss, 31 



albite or oligoclase. The quartz also occurs sparingly as ' quartz 

 vermicule.' ' In addition, the rock contains a few grains of zircon 

 and some crystals of an iron oxide, probably hematite. This 

 completes the list of constituents with the exception of a fibrous 

 mineral seemingly derived from the mica. Loss of the characteristic 

 colour and pleochroism precede the assumption of the fibrous 

 appearance. It then consists of a felted mass of crystals which are 

 of a brown colour in the aggregate by an ordinary light and exhibit 

 a rather vivid polarization. Finally, the brown colour is entirely 

 lost and the constituent crystals fray out in a brush-like way. The 

 mineral is no doubt sillimanite, and the thin section as a whole 

 forcibly recalls some of the fibrolite and cordierite gneisses of 

 Germany. The Darjiling rock is therefore a garnetiferous sillimanite 

 gneiss, in which the presence of cordierite is to be suspected. This 

 mineral no doubt does occur, but the rock has suffered slightly from 

 pressure and the felspars are usually fresh and unaltered ; facts not 

 conducive to the ready determination of such a mineral as cordierite, 

 which appears to be free from its characteristic decomposition 

 products. The inclusion of one mineral by another is a characteristic 

 feature ; e.g., rounded ' spots ' of mica are often present in the quartz, 

 or of quartz in the felspar. The larger quartz grains are much 

 cracked and the lines of fracture marked by infiltrated substances. 

 A cementing of the parts has, however, always taken place. 



A specimen from a roadside outcrop below Tigar Hill is a 

 massive rock with a distinct foliation ; ^ distinguished by a large 

 quantity of silvery-brown mica, many of the flakes measuring 



•15 inch across. In a thin section two micas are conspicuous 



a reddish brown, recalling that of some Kinzigites, and a white. 

 They occur closely associated, so that thin laminae of the brown 

 mineral are found inserted parallel or, occasionally, transversely 

 to the basal cleavage of the vvhite. Sillimanite is locally mixed up 

 with films of the latter, and possibly the rock contains cordierite. 

 Felspar is rare, and the slide contains much quartz, exhibiting very 

 irregular outlines. Some large apatite grains (-1 inch across), con- 

 siderably cracked, are also characteristic, as well as numerous small 

 zircons. A pale yellow stain, together with the absence of a basal 

 cleavage, in some of the white micas suggest this mineral is not 

 always normal, A third specimen, collected near the last, shows 

 a well-marked wrinkled structure. It contains a few irregular, but 

 not torn, garnets, and a good deal of sillimanite associated with 

 white filmy mica, as in the preceding slide. 



The rock figured is the common type of the Darjiling gneiss, and 

 in this bands of finer texture occur, one of which from the Euno-eet 

 Koad has been sliced. Macroscopically, the constituent minerals 



1 By this term is meant the thread-like intergrowth of quartz in felspar. The 

 branching threads are grouped often in a kind of bunch, not uncommonly radiatino- 

 from a point. In the examples I have met with I believe it to be an orio-inal 

 structure. ° 



- This rock differs in appearance from the others that I collected. The specimen 

 is browner in colour, the flakes of mica more conspicuous, and the foliation less 

 marked. 



