
Parr II. Sur. iii. (11.)§ 2.] OLD RED SANDSTONE. 713 
mountains and the uplands of the southern counties (Lake Caledonia). 
On the north-east it is cut off by the present coast-line from Stonehaven 
to the mouth of the Tay. On the south-west it ranges by the island of 
Arran across St. George’s Channel into Ireland, where it runs almost to 
the western sea-board, flanked on the north, as in Scotland, by hills of 
crystalline rocks, and on the south chiefly by a Lower Silurian belt. 
Another distinct and still larger basin (Lake Orcadie) lies on the north 
side of the Highlands, but only a portion of it comes within the present 
area of Scotland. It skirts the slopes of the mountains along the Moray 
Firth and the east of Ross and Sutherland, and stretches through Caith- 
ness and the Orkney Islands as far as the south of the Shetland group. 
It may possibly have been at one time continued as far as the Sognefjord 
and Dalsfjord in Norway, where red conglomerates, like those of the 
north of Scotland, occur. . There is even reason to infer that it may have 
ranged eastwards into Russia, for, as already stated, some of its most 
characteristic organisms are found also among the Devonian strata of 
that country. A third minor area of deposit (Lake Cheviot) lay on the 
south side of the southern uplands over the east of Berwickshire and the 
north of Northumberland, including the area of the Cheviot Hills. A 
fourth (Lake of Lorne) occupied a basin on the flanks of the south-west 
Highlands, which is now partly marked by the terraced volcanic hills 
of Lorne. There is sufficient diversity of lithological and paleonto- 
logical characters to show that these several areas were on the whole 
distinct basins, separated both from each other and from the sea. 
In the central basin or Lake Caledonia, the twofold division of the 
Old Red Sandstone is typically seen. The lower series of deposits 
attains a maximum depth of upwards of 20,000 feet. These strata 
everywhere present traces of shallow-water conditions. The accumula- 
tion of so great a thickness can only be explained on the supposition - 
that the subterranean movements which at first ridged up the Silurian 
sea-floor into land, enclosing separate basins, continued to deepen these 
basins until eventually enormous masses of sediment had slowly gathered 
in them. There are proofs that the subsidence was interrupted by occa- 
sional local elevations. In Lanarkshire this massive series of deposits 
passes down conformably into Upper Silurian rocks; elsewhere its base 
is concealed by later formations, or by the uncouformability with which 
different horizons rest upon the older rocks. It is covered unconform- 
ably by every formation younger than itself. It consists of reddish- 
brown or chocolate-coloured, grey, and yellow sandstones, red shales, 
grey flagstones, coarse conglomerates, and occasional bands of limestone 
and cornstone. ‘The grey flagstones and thin grey and olive shales and 
* calmstones ” are almost confined to Forfarshire, in the north-east part of 
the basin, and are known as the Arbroath flags. One of the most marked 
lithological features in; this central Scottish basin is the occurrence in it 
of prodigious masses of interbedded volcanic rocks. These, consisting of 
porphyrite-lavas, felsites, and tuffs, attain a thickness of more than 6000 
feet, and form important chains of hills, as in the Pentland, Ochil, and 
Sidlaw ranges. They lie several thousand feet above the base of the 
system, and are regularly interstratified here and there with bands of 
the ordinary sedimentary strata. They point to the outburst of 
numerous volcanic vents along the lake or inland sea in which the Lower 
Old Red Sandstone of central Scotland was laid down; and their disposi- 
