THE GEOLOGY OF ORKNEY. lxxi 
O’Groat’s beds. In Orkney they contain flows of 
dark basaltic lava and beds of greenish volcanic ash. 
The Middle or Orcadian Old Red Sandstone of 
Orkney, then, may be divided into the following 
members :— 
3. JOHN O’GRoAT’S Beps.—Principally yellow and 
red sandstones with red marls and some 
dark flags. Near their base there is a 
voleanic zone. 
2. THurso Beps.—Principally grey flagstones, 
with very occasionally thin beds of sand- 
stone. 
1. StroMNEsS Beps.—Grey flagstones, sometimes 
calcareous; thin impure limestones; sand- 
stones and grits at their base, resting on 
conglomerate beds, which in turn rest on 
the ancient granite and gneiss. 
The thickness of this pile of sediment is probably 
not less than 7000 feet, though it represents only the 
upper half of the formation, as the lower beds of 
Caithness apparently are not known in Orkney. 
They are of fresh-water origin throughout. 
After these rocks had been deposited a great 
time elapsed, during which they formed dry land 
and were exposed to the eroding action of the atmos- 
phere, rain, and streams. They were folded into 
broad arches and troughs, which were gradually 
planed across and reduced to a nearly level surface. 
Then, by degrees, another lake was formed and 
occupied a large part of Orkney and of Caithness ; in 
it sand accumulated, which now forms the highest 
