NEAK NEW GALLOWAY. 575 



parts, in the same way as the most altered shale. There are parts 

 of microscopical slides which could be matched in slides from rocks 

 farther down the hill, such as fig. 7, PI. XXIII. Fitted in with 

 these are bands and patches of a much coarser texture, in which the 

 quartz is in large pieces. These must be secondary, because they 

 often embrace large flakes of mica. Especially near the borders of 

 the less altered parts the large quartz grains form a sort of 

 ground-mass to quartz (?) granules of such regular oval or circular 

 outline as to look as if they had been acted on by a solvent. Both 

 quartz and white mica look thick and cloudy, particularly towards 

 the centre. A high power shows this to be due to the presence of 

 a multitude of fine highly-refracting needles, in bands or radiating 

 bundles, which I take to be sillimanite. All the clear grains con- 

 taining sillimanite which I have found to give a figure are uniaxial, 

 so I presume that cordierite is not present. The black mica is very 

 highly dichroic. Between crossed nicols lath-shaped sections are 

 of a light yellow in the one direction, and in the other extinguish, 

 almost as completely as tourmaline does, vibrations in the opposite 

 direction. It is noticeable that in rotating the colour changes from 

 the yellow tint to a rich brown, and that just before the maximum 

 extinction it loses all trace of brown and becomes a dull grey. 

 This mica often contains black spots ; and sometimes a high power 

 shows highly-refracting grains in the centre of these. Associated 

 with the quartz of both finer and coarser parts are occasional 

 striated felspars. These are rare ; and tourmaline, of which a 

 little has been noticed, is still rarer. 



The upper cairn (see Map, fig. 1) stands on another bold crag, 

 about halfway between the first and the granite. The rock is now 

 coarser throughout, and, if from an unknown locality, would 

 probably be described as a fine-grained gneiss. The white mica 

 is even more conspicuous than before, being present in greater 

 quantity and in larger flakes. A cleaved surface shows, besides 

 the white mica, nests of small black micas, just distinguishable as 

 such without a lens ; and between these are patches of quartz and 

 garnets. These different patches branch, and fork, and jut into 

 each other, like the counties on a map, the average greatest dimen- 

 sions being perhaps ^ inch. A cross-section is marked by the black 

 dashes which give the rock its gneissic look, and which are the 

 sections of the black mica nests. 



Microscopical slides show that more than half the bulk of the 

 rock now has the character of the coarser parts of that on which the 

 lower cairn stands. The quantity of sillimanite has consequently 

 increased. Except for this mineral the quartz is remarkably free 

 from inclusions, and thus differs from that of the most altered shale. 



The granite-junction occurs on the south side of a third boss. As 

 is shown in fig. 3, light-coloured lenticles are conspicuous in the 

 weathered cross-section. These are a foot or two long, by about 3 in. 

 broad. The main part of the rock differs from that of the upper 

 cairn in being even coarser and more distinctly banded. The white 

 micas are often as much as | inch in diameter, and form thick piles 



