LAKES IN RELATION TO GEOLOGICAL FEATURES 443 
divisions as the observer passes northwards from the Highland border 
to the crest of the Grampians, which may or may not be the original 
sequence of deposition. In Perthshire the groups are met with in 
apparent descending order, as given below : — 
12. Blair Atholl liuiestoiie and leaden schist. 
11. Quartzite (Scliicliallion). 
10. Black schist with thin limestone. 
9. Calcareous sericite schist (Ben Lawers). 
8. Garnetiferons mica-schist (Pitlochry). 
7. Loch Tay limestone. 
6. Garnetiferons mica-schist. 
5. Schistose epidotic grits (Green Beds). 
4. Ben Ledi grits and schists. 
3. Aberfoyle slates. 
2. Leny grits. 
1. Cherts, black shales, and grits of the Highland border. 
It is worthy of note that contemporaneous volcanic rocks (lavas, 
tuffs, and agglomerates) are associated with the sediments of groups 1, 
10, and 11 ; for the structures of the pillow lavas are still well preserved 
in certain areas where they have escaped deformation, notably in north 
Glen Sannox, Arran, and near Tayvallich, south of the Crinan Canal, 
in Argyllshire. Moreover, before planes of schistosity were developed 
in these Dalradian strata, they were pierced by intrusive sheets of 
basic igneous rock (gabbro and epidiorite) and acid material (granite), 
both of which shared in the movements that affected these schists. 
The age of these Dalradian sediments is still unsolved. In the 
memoir on " The Silurian Rocks of Scotland the Geological Survey 
correlated the cherts and pillow lavas of the Highland border (group 1 ) 
with the Arenig cherts and volcanic rocks of the Southern Uplands ; 
but though radiolaria have been detected in the Aberfoyle cherts, the 
evidence cannot be regarded as sufficient to prove this correlation. 
Indeed, recent researches in Anglesey and the Lleyn peninsula in North 
Wales suggest that they may belong to pre- Arenig, if not pre-Cam- 
brian, time. The presence of annelid tubes in the quartzites of Islay, 
Jura, and of the adjoining mainland is not sufficient to link these rocks 
with the Cambrian quartzites of the North-West Highlands ; for, not- 
withstanding such evidence, they might well be of older date. In 
this connection, however, it is instructive to remember that in the 
south-west of Islay there is a mass of gneiss of Lewisian type similar 
to that in the North-West Highlands, overlain unconformably by 
sedimentary strata, which have been correlated with the lower and 
middle divisions of the Torridon Sandstone. Unfortunately the 
sequence ends here, as both the gneiss and overlying sediments are 
separated by a line of disruption or thrust-plane from the quartzites 
and fucoid beds in the eastern part of the island. 
