340 LIEUT. -GEN. C. A. MCMAHON ON [May I9OO, 



to a dead stop by the ground becoming, even to an expert climber, 

 quite inaccessible. 



I propose to give, in the first instance, a summary account of the 

 granites intrusive in the Gilgit area ; then to pass on to a brief 

 survey of the Gilgit rocks, considered topographically, and to 

 conclude with some general remarks. 



(1) The Baltit Hornblende-Granite. 



This is the most basic of all the Gilgit granites. Felspar appears 

 to predominate over quartz. Eiotite in large leaves, sometimes 

 twinned, is usually abundant, and is strongly pleochroic in reddish- 

 brown and brownish-yellow tints. It polarizes vividly in blue, red, 

 and green. 



Hornblende is fairly abundant. It is pleochroic in tints of 

 green. It is usually much corroded and eaten into by the ground- 

 mass ; basal sections are sometimes idiomorphic, otherwise the 

 mineral occurs usually in allotriomorphic aggregates, often asso- 

 ciated or intergrown with biotite. In one slice all the hornblende 

 is idiomorphic. In hand- specimens it is black, and appears to 

 be a basic species rich in iron. It fuses readily in the flame of 

 a Bunsen burner to a black glass without the aid of a blowpipe, 

 and the specific gravity of an isolated fragment was found to be 

 3-368. 



The proportion of plagioclase to orthoclase varies in different 

 slices. It usually belongs to the oligoclase species, though 

 andesine is also present. Zonal structure and pericline-twinning 

 are common. Granophyric structure is sometimes prominent, and 

 microcline is occasionally present. 



Apatite is generally to be seen, and is sometimes abundant. 

 Sphene, a colourless epidote, magnetite, and a few crystals of 

 schorl also occur, and the two former are sometimes plentiful. 

 The epidote in one of the slices closely resembles white augite, and 

 it is sometimes difficult to say w 7 hich is present, especially as the 

 mineral is associated with hornblende. 



Three of the slides contain allanite. One of them shows a 

 lath-shaped crystal of this mineral which extinguishes at 40°, 

 surrounded on three sides by epidote. Another slide, which does 

 not contain any epidote, exhibits two idiomorphic prisms of allanite, 

 one of which is twinned. The mineral is brown in reflected 

 light : by transmitted light it, shows strong pleochroism, changing 

 from a reddish or chocolate-brown to brownish-yellow. The refrac- 

 tion of the mineral is strong, and it polarizes in the dull red 

 and yellow of the first of Newton's orders. The twins extinguish 

 simultaneously at about 43° to the twinning-plane. 



The macled crystal of allanite much resembles the twinned 

 allanite figured and described by Hobbs. 1 In that mineral both 

 twins extinguished simultaneously at 36° from the twinning-plaue. 

 The maximum angle of extinction in the case of a dark mineral 



1 Amer. Journ. Sci. ser. 3, vol. xxxviii (1889) p. 220. 



