Vol. 55.] IN THE REGION OP LOCH AWE. 487 



Farther westward it often passes into a rock of quite a dijfferent 

 character, pale patches consisting of fine-grained felspars and blue 

 quartz, with hornblende and chlorite occurring in foliation-planes. 

 With these occur rocks of more normal epidioritic character, 

 with plenty of hornblende and porphyritic crystals of felspar, 

 becoming occasionally fine-grained and containing epidote. Even 

 in the more basic varieties the rock has the same peculiar 

 pale patches (composed of aggregates of felspar and quartz). The 

 felspar and quartz of these aggregates are often so intergrown as 

 to form micropegmatite. Towards the westernmost part of the 

 mass the rock assumes quite a banded or gneissose character. It 

 is very massive, highly foliated, largely made up of felspar-aggregates 

 usually pinkish, small grains of quartz, dull fine-grained chlorite in 

 foliation-planes, some dark blotches of biotite in aggregates, and 

 small pegmatite-veins. In parts the rock is as acid as granite. 

 Big blotches of biotite seem to have replaced actinolite. Quartz 

 is very abundant, and quartz-veins in the rock are largely blue. 



The following are descriptions of two sections taken from this 

 rock-mass, the first of which was obtained from the rock with 

 biotite ; the second refers to that part of the mass containing horn- 

 blende : — 



(5684) Medium-grained crystalline rock with no marked foliation. 

 The white patches are as'gregates of felspar and quartz, often inter- 

 grown so as to form micropegmatite. The dark parts are aggregates 

 of biotite, epidote, and iron-ores. 



(5685) Medium-grained crystalline rock, similar in structure to 

 that just described, but with the darker parts greenish. The light- 

 coloured parts of the rock are formed of idiomorphic and more or 

 less epidotized felspar with interstitial quartz and micropegmatite. 

 The darker portions contain hornblende, chlorite, and iron-ores zoned 

 with sphene. 



Boulder-bed. — In comparing the unaltered rocks of Loch Awe 

 with the crystalline schists of the Central Highlands into which they 

 pass, I have been describing beds which are more or less continuous 

 along their line of strike. I will now describe a deposit known as 

 the Boulder-bed which occurs occasionally in the Highland series 

 along an horizon extending from Aberdeenshire to Islay. Although 

 the deposit is not continuous, it is seen so often in the Central High- 

 lands among the highly-altered schists that its absence from the 

 comparatively unaltered area of the west might be construed as 

 evidence against the identity of the Central Highland rocks with 

 those of Loch Awe. On the other hand, its occurrence in both areas 

 on similar geological horizons may be accepted as a valuable link 

 in the chain of evidence which has been submitted that these rocks, 

 notwithstanding the variable condition in their metamorphism, are 

 one and the same. 



I have stated earlier that the Ardrishaig phyllites are succeeded 

 by a series composed of black schists, limestones, and grits, and that 

 this succession holds good across the Central Highlands, whether 



