THE FRESH-WATER LOCHS OF SCOTLAND. 
41 
grits reappear and cover a wide area, extending from Ben Lomond 
north-east by Loch Katrine and the heights surrounding the head of 
Loch Voil, northwards by Ben More and westwards to Glen Falloch. 
Throughout this extensive area the strata are inclined at gentle angles : 
in marked contrast with the structure along the Highland border 
already indicated. There is here a change, over part of the area at 
least, in the lithological characters of the Ben Ledi grit group. The 
strata become more schistose and micaceous, merging in places into 
mica-schists. The accompanying geological map shows generally where 
these grits still retain their massive character and where they merge 
into mica-schists. 
The outcrop of the Loch Tay limestone is indicated on the geological 
map, from which it will be seen that this limestone, together with the 
sills of epidiorite, is traceable from the upper part of Strathyre, by the 
Kirkton glen, to Luib, in Glen Dochart. 
In addition to the great boundary fault already referred to, 
separating the lower Old Bed Sandstone from the crystalline schists, 
various faults trending N.N.E. and S.S.W. traverse the south-east 
part of the area under consideration. These are, in the main, branches 
of the great dislocation which has been traced across the Highlands 
for a distance of 60 miles, from Loch Vennachar by Loch Lubnaig and 
Loch Tay to Glen Tilt. In common with the dislocation referred to, 
the branch faults have a down-throw to the west or north-west, and 
they shift for some distance the outcrops of the strata which they 
traverse. They are truncated by the great boundary fault of the 
Highlands, and may be of pre-Old-Red-Sandstone age. 
The existing valley-system of the basin of the Teith has been carved 
out of a table-land of crystalline schists of varying hardness. Though 
there is conclusive evidence of great erosion during the successive 
glaciations of the region, yet it is clear that the present valley-system^ 
must have been developed in pre-glacial time. There is one point 
connected with the geological structure of this region which has had an 
important bearing on the evolution of the valley-system. Along the 
Highland border, as already indicated, there is a great development of 
conglomerates, coarse pebbly grits, and greywackes, belonging partly 
to the crystalline schists and partly to the Old Red Sandstone. These 
strata, being vertical or nearly so, would be much less easily eroded 
than the gently inclined schistose rocks lying to the north-west. Such 
an arrangement would naturally lead to the formation of narrow and 
comparatively flat-bottomed valleys behind rocky gorges, the latter 
being cut through the vertical beds of hard grit and conglomerate 
along the Highland border. That this remarkable structure must have 
likewise contributed to the erosion of rock-basins during the glacial 
period will become apparent on a closer examination of the geological 
structure of the area traversed by the larger lakes. 
