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 Bed 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 Bed Sandstone, both the lower and upper divisions of 
that system are represented. The Lower Old Bed 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 
Kincardineshire, 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. Bising 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 
Bed 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. 
