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 garnetiferous 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. 
F'urther 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 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 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 
