﻿392 ME. T. 0. B0SW0ETH ON METAMOEPHISM [Aug. I0,I0 y 



the strike for nearly half a mile. The best exposure of this belt is 

 at its northern end, opposite to and about 400 feet south-west from 

 the foot of Loch Assapol, in the cliff of the 100-foot raised beach. 



The rocks forming the cliff consist mainly of somewhat quartzose 

 pelitic gneiss, alternating with fine, micaceous, granulitic quartzites 

 and some thin bands of calc-silicates. There are also intrusive 

 epidiorites. 



At this place, in the central part of the belt are many quartz 

 knots and pegmatite patches, often passing gradually into the 

 pelitic gneiss. The composition of these pegmatites is variable, and 

 some are peculiar — the following being three typical examples : — 



(1) quartz and tourmaline ; 



(2) large crystals of kyanite and white felspar with some quartz ; 



(3) white felspar, tourmaline, and biotite. 



The majority of the patches are only a foot or two across, but 

 the largest is exposed along the strike for some 20 yards as a sheet 

 about 3 feet thick forming a rock-face some 15 feet high, the beds 

 here being almost vertical. The composition of this mass varies 

 from place to place. A large portion consists of about three parts 

 tourmaline and two parts white felspar, making a fairly homo- 

 geneous rock of coarse texture and slight schistosity. 



Weathered surfaces show the tourmaline as lustrous prisms ] 

 sometimes an inch in length and half an inch thick ; but, where 

 the rock is broken, the tourmaline has an unusual appearance, its 

 fractured surfaces being of a rough gritty nature and a dark olive- 

 green colour. 



In section (13976) it is seen to consist mainly of large tourmalines 

 and felspars (mostly plagioclase of low extinction, but also some in 

 which no twinning is visible). Quartz is present in a strained con- 

 dition, and the tourmaline contains sometimes as much as half its 

 bulk of quartz, in the shape of large angular inclusions with some- 

 what longitudinal arrangement, which probably are the cause of the 

 roughness of the fracture. The felspar also includes much quartz. 

 There is moreover brown biotite with deep pleochroic halos around 

 inclusions, and stout prisms of rutile are very abundant in both 

 tourmaline and felspar throughout. 



In other portions of this pegmatite, black biotite makes up about 

 half the rock, and where there is much. quartz it has the appearance 

 of a biotite-gneiss. In places large garnets are also present, as well 

 as clots of clear quartz measuring up to 1 foot across. Near the 

 margin kyanite occurs, and the rock passes into the tourmaline- 

 bearing kyanite-gneiss which borders it on each side. A few yards 

 away from the pegmatite the tourmaline becomes much less plentiful. 



The kyanite-gneiss is a rather coarse rock composed of mica, 

 felspar, and quartz, with abundant blue crystals of kyanite which 

 lie along the bedding-planes and stand out conspicuously on the 



1 W. E. Koch, Trans. Geol. Soc. Glasgow, vol. vii (1881) p. 52; also figured 

 in Heddle's 'Mineralogy of Scotland' vol. ii (1901) pi. lsxiii, fig. 1. 



