deposits, reddish-brown, yellow and grey sandstones and conglomerates,
with occasional "cornstones," and thin limestones. The grey flagstones
with shales are almost confined to Forfarshire, and are known as the
"Arbroath flags." Interbedded volcanic rocks, andesites, dacites,
diabases, with agglomerates and tuffs constitute an important feature,
and attain a thickness of 6000 ft. in the Pentland and Ochil hills. A
line of old volcanic vents may be traced in a direction roughly
parallel to the trend of the great central valley. On the northern
side of the Highlands was "Lake Orcadie," presumably much larger than
the foregoing lake, though its boundaries are not determinable. It lay
over Moray Firth and the east of Ross and Sutherland, and extended
from Caithness to the Orkney Islands and S. Shetlands. It may even
have stretched across to Norway, where similar rocks are found in
Sognefjord and Dalsfjord, and may have had communications with some
parts of northern Russia. Very characteristic of this area are the
Caithness flags, dark grey and bituminous, which, with the red
sandstones and conglomerates at their base, probably attain a
thickness of 16,000 ft. The somewhat peculiar fauna of this series led
Murchison to class the flags as Middle Devonian. In the Shetland
Islands contemporaneous volcanic rocks have been observed. Over the
west of Argyllshire lay "Lake Lorne"; here the volcanic rocks
predominate, they are intercalated with shallow-water deposits. A
similar set of rocks occupy the Cheviot district.

The upper division of the Old Red Sandstone is represented in
Shropshire and South Wales by a great series of red rocks, shales,
sandstones and marls, some 10,000 ft. thick. They contain few fossils,
and no break has yet been found in the series. In Scotland this series
was deposited in basins which correspond only partially with those of
the earlier period. They are well developed in central Scotland over
the lowlands bordering the Moray Firth. Interbedded lavas and tuffs
are found in the island of Hoy. An interesting feature of this series
is the occurrence of great crowds of fossil fishes in some localities,
notably at Dura Den in Fife. In the north of England this series rests
unconformably upon the Lower Old Red and the Silurian.

Flanking the Silurian high ground of Cumberland and Westmorland, and
also in the Lammermuir hills and in Flint an