30 J. Parkinson — The DarjiUng Gneiss. 



In this region of East AngHa we have evidence of pebbly gravels 

 in the Crag Series, in the Middle Glacial (Westleton Beds), and in 

 the Plateau Drift. 



Where one gravel is largely derived from another and is welded 

 on to it, there must often be difficulty in fixing a plane of demarca- 

 tion, as there would be (in the absence of fossils) in separating 

 other strata of identical lithological character. 



Similar occurrences, observed in Egypt by Mr. H. J. L. Beadnell, 

 where Cretaceous and Eocene clays appear to merge together, though 

 not in uninterrupted sequence, have been aptly referred to by him as 

 " unconformable passage-beds." ^ The term is at any rate a useful 

 one to bear in mind in connection with gravels, ancient and modern. 



V. — The Petrographioal Characters of the Darjiling Gneiss. 

 By John Parkinson, F.G.S. 



WELL known as Darjiling is to nearly every stray traveller 

 in India, the solid geology of the district has been left 

 almost (I believe quite) untouched since the publication of Mallet's 

 paper- in 1874. There the Darjiling gneiss was described as the 

 metamorphosed representative of the sedimentary Gondwana rocks 

 of the south, and has so remained, albeit under protest, for a dis- 

 claiming paragraph ^ appears in the Manual of the Geology of India. 

 In the report of the Committee * on the recent landslip at Darjiling 

 it is stated that the rock of the country consists of " a well foliated 

 and banded biotite gneiss with occasional lenses and deformed veins 

 of granitic rock " ; moreover, that " the foliation planes are often 

 highly contorted"; and Mallet defines it as true gneiss ''passing 

 into mica schist or an intermediate variety." These descriptions are 

 meagre, and it is hoped that the following notes on specimens 

 collected by the author may not be superfluous. 



Description. 



The gneiss outcropping near the Jelapahar Eoad, on the eastern 

 side of the ridge on which Darjiling stands, is a streaky or 

 roughly banded, rather massive rock, the planes of greatest 

 fissility glittering with black mica and containing a few pink 

 garnets the size of a small pea. In a thin section (Fig. 1) the 

 mica is the most conspicuous mineral. It is a dark green to reddish 

 brown for vibrations parallel to the basal plane, and of a deep straw 

 colour for those at right angles. The irregularly shaped garnets 

 are very pale pink in colour, cracked, and rather dirty. The colour- 

 less constituents slightly predominate, and consist largely of quartz 

 with orthoclase and plagioclase ; some crystals of the former felspar 

 exhibit a microperthitic intergrowth ; the latter is rare and apparently 



1 Eeport iii, Geol. Survey of Egypt, Cairo, 1901. 



2 Mem. Geol. Surv. Tnd., 1874, vol. xi, pt. 1. 



3 Geology of India, 2nd ed., p. 76. 



* Eeport of the Committee on the Landslip at Darjeeling, September, 1899 ; 

 published October, 1899. 



