THE GEOLOGY OF ORKNEY. lxix 
marine animals peopled its waters. Its exact limits 
cannot be defined, but we can trace the old shore 
lines through a considerable distance in the west 
and south of Caithness. It is certain also that the 
lake extended south to Inverness, Cromarty, and 
Banff, and possibly to the town of Aberdeen, for the 
red sandstones on the south side of the Moray Firth 
contain the same species of fossil fishes as the grey 
and black flags of Stromness. The Old Red Sand- 
stone of Shetland, however, apparently belongs to a 
distinct and later deposit. 
Two features of the Orcadian Old Red are especi- 
ally striking, firstly, the enormous thickness of the 
muddy sediments that gathered in the old lake, and, 
secondly, their great uniformity in character through 
what must have been a vast period of time. It was 
estimated by Sir Archibald Geikie, and recent surveys 
have confirmed his conclusions, that the pile of Old 
Red sediments ir Caithness is not less than 18,000 feet 
thick. At the bottom they rest on the crystalline and 
metamorphic rocks of the north-east Highlands, such as 
granite, gneiss, mica schist, and quartzite. The lowest 
beds are conglomerates and breccias, with red sand- 
stones and mudstones, after which the flagstones were 
laid down which underlie the town of Wick and a con- 
siderable part of the south-east of Caithness. The 
latter are supposed to have no representatives in 
Orkney, but the superjacent beds that form the 
north and north-east of Caithness extend northwards 
across the Pentland Firth. It is true that in Orkney 
the granite floor rises through the flagstones at 
Stromness, and is covered by beds of conglomerate 
