THE GEOLOGY OF G3 =] 
HE geology of Orkney in its main outlines is 
not of a very complex type. Practically the 
entire county is underlain by the Old Red Sandstone, 
and of this geological formation, two divisions are 
represented—the Middle (also known as the Oreadian) 
and the Upper Old Red Sandstone. Older rocks, 
principally granite and mica schist, form a small area 
near Stromness; and especially in Hoy and Deerness 
there are local outcrops of volcanic material. In 
these respects Orkney bears a close resemblance to 
Caithness, and in both counties flagstone is by far 
the most wide-spread rock. The flat contours and 
gentle slopes that characterise the landscapes are 
equally well marked in both counties. Shetland, on 
the other hand, is far more varied in its scenery and 
eeolovical structure, though on both sides of Shetland 
the Old Red Sandstone spreads over considerable 
areas. | 
The Oreadian or Middle Old Red Sandstone, to 
which the flagstones belong, is known to be a fresh- 
water deposit laid down in extensive lakes which 
were inhabited by many species of fishes, though no 
