Vol. 60.] METAMORPHISM IN THE LOCH-LOMOND DISTRICT. 17 



minerals serves to class the rock as a phyllite. The developing 

 schistosity is along the planes of cleavage. Reconstruction of the 

 matrix under constructive metamorphism is at a very low phase. 



(c) The Beinn-Ledi Group. 



Still proceeding north-westward, we reach the Beinn-Ledi Group 

 of grits, schists, and greywackes. These rocks, where folded into a 

 vertical isocline, have as a rule suffered little from shearing-stresses, 

 especially where coarse and gritty in texture and highly siliceous in 

 composition ; the finer and more felspathic bands, on the other hand, 

 present a more favourable field for the action of metamorphic 

 processes, and are accordingly more greatly affected. 



A specimen (3679) taken from the Trossachs, a quarter of a mile 

 east of Loch Katrine, shows the state of a fairly fine grit in a 

 vertically-folded area. The matrix is mainly composed of granulitic 

 quartz, with some cloudy indeterminate material and sericitic mica. 

 The pebbles are chiefly of quartz, though a number of small striated 

 felspars are visible. The felspar-pebbles appear to have been frac- 

 tured or crushed more than the quartz, and there is a little calcite 

 present — an important point, which will be referred to later. There 

 is very little sign of shearing or orientation of the pebbles ; they 

 lie at all angles to the planes of schistosity, but show peripheral 

 granulitization and a tendency to merge into the matrix. There 

 is* no recrystallization of the matrix to obscure the original 

 planes of bedding, which coincide with the schistosity ; but much 

 of the granulitization may possibly be due to incipient thermo- 

 metamorphism. On the whole, the rock shows fewer signs of 

 dynamic metamorphism than the last, and very little constructive 

 metamorphism. 



To examine the effects of metamorphism upon the rocks at 

 about this horizon — high up in the Beinn-Ledi Group, — a series of 

 specimens was collected from the western shore of Loch Lomond, 

 all being taken at approximately the same level, and being (as nearly 

 as it was possible to ascertain) from the same horizon. It has 

 been mentioned before that in this area, west of Loch Lomond, rocks 

 of the Beinn-Ledi Group occupy the surface from Luss to Ardlui. 

 The next specimen taken comes from lludha Mor, beyond the belt 

 of vertical folding and where the folds are rapidly becoming flatter 

 and of small amplitude. This specimen (8984, PI. Ill, figs. 1 & 2) 

 shows, as might be expected, a much advanced stage in the meta- 

 morphism. It has been a coarse siliceous grit, but now, although 

 the larger pebbles are still distinct, the development of planes of 

 schistosity has cut up the rock into elongated phacoids, with 

 micaceous folia separating them. Under the microscope the matrix 

 is seen to consist of granulitic quartz, sericitic mica, biotite, and a 

 few grains of sphene ; the pebbles are of quartz and decomposed 

 felspar. The quartz-pebbles are sometimes completely granulitized 

 and merging into the matrix ; others are only peripherally granu- 

 litized, but show strain-shadows and drawn-out * tails ' of granulitic 



Q. J. G. S. No. 237. c 



