42 BATHYMETRICAL SURVEY OF 



In the case of Loch Katrine, which is the largest and deepest of the 

 lochs under consideration, there is a great rocky barrier at its outlet due 

 to the Ben Ledi grits. Here they form a belt over a mile in breadth, 

 and give rise to the rugged scenery so characteristic of that region. 

 They appear on the crags of the Trossachs at the mouth of the loch, on 

 the crest and slopes of Ben Venue (2393 feet), 011 Ben Bhreac (2295 feet), 

 and on the heights round Ben An (1326 feet). The strike of these hard 

 and durable strata is E.N.E. and W.S.W. that is, at right angles to 

 the outlet of the loch, and the beds are vertical or highly inclined. 



The potency of the Ben Ledi grits as a rocky barrier must have been 

 considerably increased by the development of epidotic grits or " Green 

 Beds " lying immediately to the north. The latter, though not so 

 massive as the Ben Ledi grits, are hard and durable ; they are repeated 

 by a series of compound folds for nearly a mile across the strike, their 

 northern limit being near Brenachoil Lodge. Their trend is likewise 

 north-east and south-west, and the beds are vertical or highly inclined. 



On both sides of Loch Katrine above Brenachoil Lodge the geological 

 structure is widely different, for in this area the Ben Ledi grits, grey- 

 wackes, and slates reappear in a highly schistose form, the strata 

 dipping generally at low angles to the south-east. Over much of this 

 region, as already indicated, the altered sediments merge into mica- 

 schists owing to the development of mica. It is obvious that these 

 materials would yield more readily to the agents of denudation than 

 the massive pebbly grits of Ben Venue and the Trossachs. 



Loch Achray, which lies about a mile to the east of the outlet of 

 Loch Katrine, is only about 88 feet below the level of the latter loch. 

 A powerful fault or dislocation, trending north-east and south-west, 

 crosses the head of the loch near the Trossachs Hotel, which brings the 

 massive Ben Ledi grits to the west in contact with slates to the east. 

 It is a true rock basin which has been excavated mainly in the group 

 of less durable slates. 



Loch Vennachar is crossed by the great boundary fault, already 

 referred to, along the Highland border, the floor of the eastern portion 

 being composed of Old Red Sandstone conglomerate, while that of the 

 western part is formed of grits and slates belonging to the crystalline 

 schists. Though there is a covering of drift on both sides of the lower 

 part of the loch, still this sheet of water forms a true rock basin, for the 

 Old Red conglomerate is exposed in the river about 1200 yards below 

 the outlet. 



Loch Drunkie presents several interesting geological features. On 

 referring to the map it will be seen that an arm of this loch runs nearly 

 east and west for upwards of half a mile ; the northern margin is com- 

 posed of massive grits, while the southern margin and probably the floor 

 of this branch of the loch is formed of less durable slates. Another 

 arm of this lake runs N.N.E., in the direction of Loch Vemiachar, the 



