16 -ME. E. H. CUNXIXGHAM-CEAIG ON [Feb. I904, 



of granulitic material drawn out in the direction of schistosity. 

 The felspar-pebbles are either fractured or apparently unaffected, 

 but are often much decomposed. The matrix is also cloudy and 

 decomposed; it consists of granulitic quartz, a little chlorite and 

 sericitic mica, and iron-ores, and it has never attacked the pebbles, 

 even where they are somewhat ragged in outline. A slide of 

 coarse grit 1 (8983, PI. II, fig. 1) from Craignahuillie, south of 

 Luss, is a fair instance of the state of metamorphism attained 

 in this group. A finer grit from Creachan Hill, south of Luss, 

 shows the pebbles with ragged ends, and the complete granuli- 

 tization of some of them. An alkali -felspar (much decomposed) 

 is present in this rock, and a small patch of microcline has been 

 preserved in a quartz-pebble. The matrix is chloritic and sericitic. 

 The preservation of the microcline by being included in a fragment 

 of quartz is significant, as suggesting that alkali-felspar may have 

 been present in greater quantit}' in the original rock but has been 

 destroyed in the dynamic metamorphism, giving rise to the sericitic 

 mica of the matrix. Clastic micas may be occasionally detected 

 in the finer grits. These beds occur, as has been stated, in highly- 

 compressed vertical folds, and it is evident that the phase of 

 dynamic metamorphism is not very high, while constructive meta- 

 morphism can hardly be said to have commenced, its only effects 

 being the meagre development of sericitic mica and chlorite, which 

 may also be partly of clastic origin. 



{b) The Aberfoil-Slate Group. 



Passing north-westward, and ascending in the series, we come to 

 the Aberfoil Slates and slaty grits, a series of fine sediments which 

 are much more liable to dynamic alteration from their comparative 

 softness, and also to constructive metamorphism from their more 

 complex composition. A section of slate (2567), from the head of 

 Glen Fruin, shows a considerable development of more or less 

 indeterminable micaceous minerals, chiefly sericitic mica and 

 chlorite, along the cleavage-planes, which cross the bedding at 

 a high angle. The bedding is marked by the presence of minute 

 aggregates of quartz, and one distinct pebble is noticeable. How 

 much of the micaceous constituents can be said to be due to 

 constructive metamorphism it is impossible to say, but there has 

 evidently been no crystallization on more than a very minute 

 scale ; the rock has been deformed, and the constituents rearranged 

 by dynamic metamorphism, but the constructive metamorphism is 

 still at a minimum. 



Another section (2568) from Kowmore, Garelochhead, shows an 

 originally fine gritty rock assuming the character of a phyllite. 

 Much drawn-out phacoids of quartz and a little plagioclase-felspar 

 show that the rock was originally gritty, but a good deal of the felspar 

 has probably been destroyed, and the development of micaceous 



1 The numbers are those of the slides in the collection of the Geological 

 .Survey of Scotland. 



