^Ol. 55.] IN THE REGION OF LOCH AWE. 485 



^iharacters of hard and soft alternations. The micaceous partings 

 :and the quartzite-bands have, as it were, become welded into a solid 

 homogeneous mass. The dark micaceous alternations, when the 

 rock is in this condition, impart a banded structure and a bluish 

 coloration to the rock. The enormous quantity of secondary biotite 

 which is scattered through both the quartzose and micaceous zones 

 still further darkens them. These grits and quartzites have 

 passed into rocks which are gneissose in character ; the more 

 siliceous, however, remain as quartzites and quartzite-schist. 

 These granulitic and gneissose rocks occupy the area to the north 

 of the Strath of Orchy till they are cut off to the westward by the 

 Cruachan granite. From this Strath of Orchy area they stretch away 

 north-eastward into the Central Highlands as quartzites, gneissose 

 flagstones, biotite-gneisses, etc., and in the Central Highland area, 

 so far as is known at present, they retain this highly crystalline 

 character without reverting to the comparatively unaltered condition 

 in which they are found in the Loch Awe basin. 



Biotite, garnets, actinolite, small tourmalines, epidote, and red 

 felspar are common as secondary minerals in this highly-altered 

 area. The biotite has been one of the latest minerals to crystal- 

 lize, and has been generally unmoved ; the same may be said 

 for the tourmalines and red felspars, though these latter may be 

 sometimes broken. The garnets and actinolites attain considerable 

 dimensions ; garnets occur up to ^ inch in size, and actinolites 

 are in blade-like forms sometimes 1 inch long. The garnets and 

 actinolites are commonest in the micaceous beds. The former, 

 too, have a habit of occurring thickly in certain zones ; for 

 instance, one bed was observed in the neighbourhood of Glen 

 Orchy 6 feet wide, literally studded with garnets | inch in size 

 or even larger. In this case the foliation was clearly seen to curve 

 round the garnets, which, besides being crushed, were often cleaved 

 parallel to the foliation-planes of the rock. In many cases, 

 however, these garnets show no sign of movement, but preserve 

 their external crystalline outline. Small black tourmalines about 

 I inch long are also seen quite unmoved. 



Eed pegmatites are seen in the area lying between Glen Orchy 

 and the Cruachan granite. They occur as red felspathic material, 

 filling up joint-planes, and are common in quartz-veins. These 

 felspathic veins are always unmoved ; they belong to the same set of 

 phenomena as the introduction of the secondary red felspars among 

 the quartzose rocks of this area. This felspathization is clearly 

 connected with the presence of the Cruachan granite. The 

 quartzose schists on the western side of Gleann Strae into which 

 the granite has intruded display this condition in a marked 

 degree. The felspathization has proceeded so far that the whole 

 hillside presents a pinkish hue, in place of the normal pale- 

 grey colour of these quartzite features. Near the granite-margin, 

 isolated crystals of red felspar appear sometimes up to 1 inch 

 in length, while felspars | inch long are quite common. With 

 these larger crystals smaller felspar-grains are associated, and as 



