THE FRESH-WATER LOCHS OF SCOTLAND. 
51 
small shallow basin, about 41 feet deep (Lochan Dubh), floored by 
schistose grits, which is traversed by a fault trending north-east and 
south-west, with a downthrow to the east. Across the mouth of this 
basin a band of massive, pebbly grits of the Ben Ledi type has been 
traced. 
A reference to the geological map will show that the direction of the 
ice-flow during the great glaciation coincides generally with the trend 
of the loch, striae being found on the rocky islands as well as round the 
margin of the lake. The evidence supplied by the soundings tends to 
support the theory that the basin-shaped hollow has been eroded by 
ice-action. The dislocations referred to above have doubtless produced 
local modifications of the floor of Loch Chon and of that of the small 
basin (Lochan Dubh), but they do not account for the excavation of the 
basin. 
Loch Ard . — Loch Ard is also a true rock basin, which lies along the 
outcrop of a belt of slates between two bands of grit, the deepest part 
of the loch, as proved by the soundings, coinciding with the outcrop of 
the slates. 
From the Mill of Chon downwards to the head of the lake there is 
a small alluvial flat pointing to the former extension of the loch in that 
direction. At the upper end the soundings show that the average depth 
is 25 feet, with the exception of one small depression opposite Ledard 
burn, reaching 57 feet in depth. Eastwards, where the loch becomes 
narrower, the depth increases. The basin enclosed by the 50-feet 
contour-line is IJ miles long, while that surrounded by the 100 -feet 
contour-line is three-quarters of a mile in length, the deepest sounding 
being 107 feet. 
The dislocation, with a downthrow to the east, that crosses the loch 
in line with Allt-na-Sgeith in a north-east and south-west direction has 
not produced any local modification of the floor of the lake, if we may 
judge by the soundings. The 100-feet basin crosses this fault without 
any apparent increase in depth on the side of the downthrow, which is 
probably due to the fact that the dislocation brings slates into contact 
with slates. The band of massive grit which forms for a long distance 
the southern margin of the loch evidently acted as a barrier during the 
period of glacial erosion. Crossing the lake at Briedach, this band of 
grit forms the promontory south-east of Glashart. 
On referring to the geological map, it will be seen that the band of 
grit just described is followed southwards by slates, the outcrop of 
which coincides with an expansion of the loch at its outlet, the deepest 
sounding being 52 feet. About 600 yards to the east of the outlet the 
trend of the latter belt of slates is E.N.E., and here occurs another 
small basin upwards of 30 feet in depth. 
No ice-markings have been found round the margin of the loch or 
near it ; but about half a mile to the south of the upper end of the lake 
