40 
BATHYMETRICAL SURVEY OF 
belong to the lower Old Hed Sandstone formation, comprising, next the 
fault, andesitic lavas and agglomerates well seen in the Kelty water. 
Further to the south-east there is a broad belt of conglomerate arranged 
in beds, which are inverted or vertical near the fault, and as the 
observer approaches the plain they dip towards the south-east and 
pass underneath the overlying red sandstones. 
On the north-west side of this great boundary fault of the High- 
lands there is a narrow strip of sedimentary rocks about half a mile in 
breadth, referred provisionally to the Arenig division of the Silurian 
system, and consisting of red and black shales, radiolarian cherts, lime- 
stones, and grits. 
To the north of this belt of doubtful strata, the whole of the area 
included in the geological map accompanying this paper is occupied by 
rocks grouped under the general term of the crystalline schists of the 
Highlands. The latter are arranged in a definite order, but as yet it 
is uncertain whether it indicates the original sequence of deposition. 
The groups are here given in apparent descending order — 
7. Garnetiferous mica-schists. 
6. Loch Tay limestone with sills of epidiorite. 
5. Mica-schists with sills of epidiorite. 
4. Schistose epidotic grits (“ Green Beds 
3. Ben Ledi grits, massive and sometimes schistose. 
2. Aberfoil slates with subordinate bands of grit. 
1. Leny and Aberfoil grit. 
For a distance of about 5 miles northwards from the great boundary 
fault, the members of groups 1 to 4 are arranged in more or less parallel 
belts or strips running south-west and north-east, the strata dipping 
at high angles to the north-west. The groups appear in consecutive 
order, the Leny and Aberfoil grit being exposed immediately to the 
north of the doubtful Arenig rocks, while the Aberfoil slates and Ben 
Ledi grits appear successively to the north. The schistose epidotic 
grits (group 4), which lie apparently at the top of the Ben Ledi grits, 
are developed still further to the north, being traceable from a point 
not far to the south of Ben Lomond, north-east by Loch Chon and the 
lower part of Loch Katrine, thence across the hills to Strathyre and 
Loch Voil. From the Braes of Balquhidder they can be followed north- 
wards to Glen Dochart, and they reappear in Glen Falloch in the 
extreme north-west part of the map. At the head of Loch Lubnaig 
and in the Braes of Balquhidder sills or intrusive sheets of epidiorite 
occur at no great distance from the ‘‘ Green Beds.” 
In the belt between Loch Chon and Loch Lubnaig the Green 
Beds,” together with the Ben Ledi grits, form a series of compound 
synclinal folds, the strata being inclined at high angles. To the north 
and west of the “ Green Beds ” the representatives of the Ben Ledi 
