Vol. 60".] METAMORPHISM IX THE LOCH-LOMOND DISTRICT. 15 



The north-western margin of the trough gives evidence of low-lying 

 folds which are gradually losing their sharpness, and the rocks 

 emerging from beneath the trough exhibit this to an even greater 

 extent, so that very rapidly all trace of regular folding is lost. 

 Rolling and puckered folding of no great depth is the rule, and con- 

 tinues north-westward to the neighbourhood of Ardlui at the head 

 of Loch Lomond, where evidence of more regular folding, the margin 

 of the great Highland fan-structure to the north, begins to appear. 



The structure, then, from the great trough to the north-western 

 corner of the sheet, is practically horizontal, the minor rlexuring 

 and crumpling of the beds being of little account, and the surface 

 is occupied by rocks of the Beiun-Ledi Group. This fact was only 

 arrived at by careful examination of a great part of the area, and 

 the identification of outliers of the coarsest bed of Beinn-Ledi Grit 

 at various points : as, for example. An Garadh north of Loch Katrine, 

 Beinu Lomond. andBeinn Bhreac and Tullich Hill in Dumbartonshire. 

 Still more important were the recognition of an outlier of the Green 

 Beds on Cruinn Bheinn, north of Loch Katrine, at a distance of 

 2 miles from the margin of the great trough, and the identification 

 of the Green Beds again at Ardlui, in a very highly- altered state. 



These points having established the stratigraphy and the general 

 structure by actual mapping, we are now enabled to consider the 

 bearing of the structure upon the general and progressive meta- 

 morphism of the area. 



V. Petrographical Descriptions. 



A large number of microscope-slides from the area have been 

 examined, and in the following descriptions selected slides and 

 specimens from different parts of the area form the basis of the 

 account of the various phases of metamoiphism to be considered ; a 

 series collected from the western shore of Loch Lomond to 

 illustrate the progressive metamorphism, for the Explanation of 

 Sheet 38 of the 1-inch Geological Survey Map of Scotland. is 

 included. - 



(a) The Leny-Grit Group. 



The Leny Grits appear to differ very little in character, from 

 whatever part of the area they are collected. They consist 

 essentially of pebbles of quartz and felspar (mostly oligoclase) 

 in a fine matrix which is usually more or less chloritic, and 

 partakes more of the nature of an ordinary clastic than a crys- 

 talline rock. The matrix exhibits a distinct schistosity ; but 

 the pebbles, especially where the grit is coarse, are little affected : 

 strain-shadows in the quartz and peripheral granulation are often 

 the only signs of dynamic metamorphism. The smaller pebbles, 

 however, may be completely granuhtized, or may show ' tails ' 



1 I am indebted to Air. J. D. Falconer, of the University of Edinburgh, for 

 very kindly preparing the microphotographs which have been used to illustrate 

 this paper. 



