448 THE FKESH-WATER LOCHS OF SCOTLAND 
The deposits of the lower division may be arranged in three groups : 
(1) a lovver, consisting of conglomerates, sandstones, and flags with no 
volcanic rocks ; (2) a middle, composed almost wholly of lavas, 
tuffs, and agglomerates ; (3) an upper, consisting of conglomerates, 
sandstones, flags, and mudstones. The members of the upper group 
are splendidly developed in the centre of a great trough extending 
from Stonehaven by the Braes of Doune to near Dry men — a distance 
of ] 00 miles ; while the lavas and ashes of the middle group rise from 
underneath these and form a prominent arch in the Sidlaws and 
Ochils. The members of the lower group are exposed on the coast at 
Stonehaven, where, at their northern limit, they are truncated by a 
powerful fault which brings them into conjunction with the meta- 
morphic rocks of the Highlands. As already indicated, this great 
dislocation stretches from the Kincardineshire coast to the Firth of 
Clyde, and through part of its course brings different members of 
this formation against each 'other. On the north side of the fault, 
between Criefl* and Cortachy, there is a development of coarse trappean 
conglomerates with thin beds of lava occupying the horizon of the 
volcanic series and resting unconformably on the metamorphic rocks, 
while the underlying beds are absent or sparingly represented. It is 
apparent from this overlapping of the strata that there must have been 
a gradual depression of the Highland barrier, and that as the waters 
of the lake crept northwards the crystalline schists of the Highlands 
were buried under the accumulating sediments of the higher groups. 
The foregoing subdivisions are conspicuously developed in the belt 
that borders the northern margin of the Southern Uplands. In this 
case also the Lower Old Red Sandstone is bounded by a great fracture 
extending from Midlothian to the Firth of Clyde, whereby this 
formation has slipped downwards against the Silurian tableland to 
the south. In the Pentland Hills the volcanic series, comprising ande- 
sites, rhyolites, and tuffs, forms conspicuous features in the landscape 
and is traceable at intervals along the belt south-westwards into 
Ayrshire. Beyond the Silurian tableland, in the Cheviots, these 
volcanic rocks are well developed, and they form a broad plateau in 
Lorne, Argyllshire, where they are associated with sediments which 
have yielded fish-remains of Lower Old Red Sandstone age. They like- 
wise appear in the Glencoe region and on the crest of Ben Nevis. 
A striking feature of this period is the extent and variety of the 
plutonic intrusions (granite, diorite) in the Highlands and Southern 
Uplands, to which reference has already been made. 
In the crreat northern area, where the middle or Orcadian series 
of the Moray Firth, Caithness, Orkney, and Shetland appears, there is 
a marked divergence in the character of the strata and the fish fauna 
from that on the south side of the Grampians. Murchison clearly 
