THE FRESH- WATER LOCHS OF SCOTLAND. 137 



of Glen Garry, and Loch Lyon, near the head of Glen Lyon, likewise 

 occur along lines of fault. In each of these cases, the long axis of the 

 loch coincides with the course of a more or less powerful dislocation, 

 which has been traced for miles. 



The following instances might be discussed in relation to the 

 question of the glacial origin of rock basins : Loch Rannoch, Loch 

 Tummel, Loch Tay, Loch Earn, Loch lubhair, and Loch Dochart. Of 

 these, the first four have been previously described by our colleague, 

 Mr. J. S. Grant Wilson, in the Scottish Geographical Magazine for 

 May, 1888, in connection with the soundings made by him in the 

 course of the geological survey of the district. It is not necessary, 

 therefore, to give in detail the evidence in support of the view that 

 these lochs, with the exception of Loch Tay, have been eroded by ice- 

 action. His soundings have been, as a rule, confirmed by Sir John 

 Murray and his staff. 



Loch Raimoch is a fine instance of a rock basin, for though, at the 

 lower end, the river Tummel, which issues from the loch, flows along 

 an alluvial flat for a distance of 3 miles as far as Dun Alastair, a rocky 

 barrier appears at the latter point in the river and on the hill slopes. 

 Near the foot of the loch, on either side of the valley, there is a 

 prominent mass of high ground, culminating in Schichallioii (3547 feet) 

 and Beinn a' Chuallaich (2925 feet). The streams draining this high 

 ground to the north and south have silted up the loch at the lower end, 

 and have produced the long stretch of alluvium between Kinloch 

 Rannoch and Dun Alastair. The longitudinal section of Loch Rannoch 

 shows that the loch gradually deepens from the west margin towards 

 the centre and lower end. The soundings further show that between 

 the mouth of the Dall burn and the foot of the loch there are three 

 small basins, each over 400 feet in depth. The deepest sounding 440 

 feet is in the centre of the largest and most easterly of these three 

 basins, and within 2 miles of Kinloch Rannoch. On referring to the 

 geological map it will be seen that the Loch Garry fault crosses Loch 

 Rannoch near Dall in a S.S.W. direction; and, notwithstanding the 

 fact that the downthrow side of this fault is towards the west, yet the 

 deepest sounding is found on the upthrow side. 



Loch Tummel is another typical example of a rock basin, the rocky 

 barrier appearing in the stream and on the hill slopes at Allean House, 

 about a mile below the mouth of the lake. For several miles down- 

 stream, as far as Faskally, the Tummel cuts through solid rock, 

 composed mainly of the Perthshire quartzite, with bands of black 

 schist. This loch has had originally a greater extension westwards, 

 for it has been silted up by alluvial matter deposited by the streams. 

 It is about 2 miles long, and the soundings show that it forms three 

 separate basins of no great depth, the deepest sounding of the western 

 basin being 128 feet; of the central, 119 feet; of the eastern, 99 feet. 



