LAKES IN RELATION TO GEOLOGICAL FEATURES 479 
descriptive of this lake we have stated that for a distance of four 
miles west from Brenachoil Lodge to Stronachlachar — about half of the 
total length of the loch — the lake has a comparatively flat bottom 
enclosed by the 400-feet contour line. The deepest sounding is 495 
feet, which is situated at the eastern limit of this basin, nearly due 
south of Brenachoil ; and the chart shows that the soundings gradually 
increase in depth eastward to Brenachoil. The position of the 
deepest sounding is of special interest ; for the strata which there 
occupy the floor of the lake consist of schistose micaceous grits in 
front of the massive Ben Ledi grits and epidotic grits (Green Beds) 
which form the great rocky barrier at and above the outlet of the lake. 
A study of an orographical map shows that the depressions 
containing these lakes are connected by low passes with valleys lying 
farther to the north ; and hence, during the period of confluent glaciers, 
the volume of ice would be greatly increased and maintained for a 
considerable time. 
Another series of valley rock-basins illustrating the relation of 
geological structure to diff'erential ice-erosion occurs in the North- 
West Highlands, where the lakes lie in weaker Torridonian strata and 
the barrier consists of the harder Lewisian Gneiss. In the Coigach 
district of West Ross-shire a chain of lakes — viz. Lochs Bad a 
Ghaill, Rudha na h'Aclise, and Lurgain — is ponded by a ridge of 
Lewisian Gneiss once deeply buried beneath the Torridon Sandstone 
till the denuding agents that formed the valley exposed its top. 
Loch a' Bhealaich and Loch Damh, on the north and south sides of 
Loch Torridon respectively, lie in Torridonian strata with a similar 
barrier of Lewisian Gneiss. The sea-lochs Little Loch Broom and 
Upper Loch Torridon, which are small fjord basins, fall into the same 
category. 
Loch Shin is an excellent example of a lake ponded by a rocky 
barrier. It lies more or less along the strike of the crystalline schists 
of the Moine series in the old consequent valley of the Shin, and its 
barrier consists of a belt of the same rocks invaded and indurated by 
a plexus of granite intrusions which have rendered them highly 
resistant. 
No less striking instances are those elongated rock-basins in the 
valleys of Coruisk (Loch Coruisk) and Camasunary (Loch na Creubh- 
aich) in Skye, which have been fully described by Mr Harker. The 
determining condition in both cases was the same, a marked 
constriction of the valley towards its lower end, which must have 
occasioned a heaping up of the ice. Mr Harker states that in 
Coruisk the constriction was caused by the Sgur Dubh ridge running 
out eastward from the main range ; while in the Camasunary valley 
the same effect was produced by the convergence southward of the 
