Ixxii FLORA ORCADENSIS. 
hills of the county. The yellow sandstones of the 
hills of Hoy belong to the Upper Old Red Sandstone, 
and are the deposits of this second lake. Similar 
rocks occur also at Dunnet Head on the south side of 
the Pentland Firth. Their beds are nearly horizontal, 
but below them the grey flagstones of the Stromness 
beds are seen dipping steeply to the west in Hoy 
Sound. The Upper Old Red Sandstone rests on the 
tilted edges of the Middle Old Red beds, which had 
been worn down to form a platform; the unconform- 
ability between the two series is consequently very 
marked. | 
From the point of view of the botanist, the 
geological structure of the Orkneys and the history 
of the rocks are of less importance than the nature of 
soils and subsoils and the configuration of the county. 
By far the commonest of the Oreadian rocks is flag- 
stone, a type which is found in large development 
only in Orkney, in Caithness, and on the east side of 
Shetland. The Orcadian flagstones are fine grained 
dark grey rocks, which, though not hard, are often 
distinetly brittle. They contain a large percentage of 
clay and fine worn particles of white mica, but most 
of them have an admixture of sand, and they are 
nearly always somewhat calcareous. Their weathered 
surfaces are generally yellow or brown, and on de- 
composition they become covered with a layer of clay 
from which the lime has been removed, while the iron 
has become completely oxidised. The flagstones, 
however, are far from uniform in character. Many 
are quite arenaceous, and may be described as argil- 
laceous sandstones. Occasionally we find beds of 
