THE FRESH -WATER LOCHS OF SCOTLAND. 133 



plateau north of that valley, and far to the south in Glen Lednoch 

 between Comrie and Loch Tay. Several of these igneous intrusions 

 consist partly of diorite and partly of granite, the more basic type being 

 erupted prior to the more acid. 



At the western margin of the basin on the lofty peaks of the Black 

 Mount forest there is a terraced plateau of contemporaneous igneous 

 rocks of Lower Old Red Sandstone age, pointing to the former 

 extension of this volcanic series, the distribution of which is of 

 importance in connection with the glaciation. These are pierced by 

 plutonic rocks (granite), which have produced a certain amount of 

 contact alteration in the lavas. 



In the lower part of the basin of the Tay, which is almost wholly 

 occupied by Old Red Sandstone, both the lower and upper divisions of 

 that system are represented. The Lower Old Red Sandstone has by 

 far the greater development, being divisible into a lower volcanic series 

 and an overlying group of sandstones, conglomerates, and marls. Two 

 great flexures cross the basin in a north-east and south-west direction, 

 roughly parallel with the fault along the Highland border. One of 

 these flexures forms a broad arch, exposing a great series of contem- 

 poraneous volcanic rocks in the Ochils and the Sidlaws ; the other forms 

 a great trough, in line with the valley of Strathmore, containing the 

 highest members of this division in the basin of the Tay. The anti- 

 clinal fold is prolonged far to the north-east into Forfarshire and 

 Kiiicardineshire, where sandstones and flags appear in the crest of the 

 arch. In the Ochils the total thickness of lavas, tuffs, and agglomerates 

 in the north limb of the fold is about 6000 feet, and they were probably 

 deposited on a gradually sinking area ; nevertheless, some of the volcanic 

 cones may have ultimately appeared above the level of the water and 

 become subaerial. Rising out from underneath the overlying sand- 

 stones and marls, along the Highland border, the volcanic series again 

 appears, though in a very attenuated form, consisting of andesitic lavas, 

 which are associated with coarse conglomerates containing pebbles of 

 volcanic rocks. Indeed, the lavas, conglomerates, and sandstones occur 

 on the north side of the fault at Blairgowrie, and again at Crieff, where 

 they rest unconformably on the metamorphic rocks. The broad tract 

 of low ground between the Sidlaws and the Highland border has been 

 carved out of the softer sandstones and marls overlying the volcanic 

 series. The river Isla, when it enters the area occupied by this 

 overlying sedimentary series, is deflected towards the south-west till it 

 joins the Tay. 



The long interval which elapsed between the Lower and Upper Old 

 Red Sandstone periods was marked by great denudation of the members 

 of the lower division of that system. The strata were thrown into 

 anticlinal and synclinal folds, the axes of which are roughly parallel 

 with the trend of the fault along the Highland border. And further, 



