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 Red 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 Gflen 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. 



