138 BATHYMETRICAL SURVEY OF 



Where these slopes and barriers appear, streams enter the lake from 

 the south, which have given rise to cones projecting for some distance 

 into the loch. It is probable, however, that they may be formed partly 

 of solid rock. Judging from the evidence round the sides of Loch 

 Tummel, the floor of that loch consists mainly of black schist, with 

 infolds of the lower part of the quartzite. 



Loch Earn may be described as the best instance of a typical rock 

 basin within the catchment area of the Tay. Upwards of 6 miles long 

 and about three-quarters of a mile broad, the soundings show that it 

 is a simple basin. The deepest sounding 287 feet occurs about half- 

 way down the loch. The Loch Tay fault crosses the lake about a mile 

 from the upper or western end; and along its course there is a small 

 basin, the greatest depth of which is 240 feet. West of this fault, the 

 floor of the loch is composed of the Loch Tay limestone and the under- 

 lying garnetif erous mica-schists ; east of it, for some distance, the lake 

 lies obliquely across the strike of the schists overlying the Green Beds 

 and the Green Beds themselves; while at the foot of the loch the Ben 

 Ledi grits appear as a rocky barrier crossing the valley at St. Fillans. 



Lochs lubhair and Dochart may be cited as further instances of rock 

 basins. Originally forming one sheet of water, they have been isolated 

 by alluvial matter brought down by the stream that drains the great 

 corrie west of Ben More. The deepest sounding of Loch lubhair 65 

 feet is near the foot. Roches moutonnees appear in that lake, both 

 about the middle and near the foot. Loch Dochart is being rapidly 

 silted up; indeed, it must formerly have extended for 3 miles up the 

 valley of Strath Fillan. The deepest sounding of Loch Dochart is 

 11 feet. 



Further down Glen Dochart there is a strip of alluvium about 5 

 miles long, between Luib station and Easter Lix, which may probably 

 represent a silted-up rock basin. 



Loch Tay presents certain features which differentiate it from the 

 rock basins already described. There is no rocky barrier close to the 

 lake; the Loch Tay fault runs along the course of the lake for a 

 distance of 5J miles from Ardeonaig to Stronfearnan ; the greatest 

 depth, which is 508 feet, lies on the downthrow side of this dislocation ; 

 and finally there is a basin 12 miles long, the whole of which is below 

 the level of the sea. The first appearance of solid rock in the bed of the 

 Tay is north of Grandtully castle, about 8 miles below the foot of the 

 loch, where mica-schists appear, belonging to the group of the Ben 

 Ledi grits. For a distance of 1J miles below this point to near 

 Ballinluig village the river flows at intervals over rocky ledges. There 

 can be no doubt that the deflection of the original valley of the Tay 

 between Ardeonaig and Stronfearnan was due to the Loch Tay fault, 

 whereby the Loch Tay limestone and associated schists on its western 

 side were brought into conjunction with the intrusive igneous masses 



