408 ; PROFESSOR GEIKIE ON THE 
I have myself twice visited Orkney, accompanied on each occasion by my 
colleague in the Geological Survey, Mr B. N. Peacu, with whose co-operation all 
my observations were made. My object on the first visit was to ascertain 
whether the yellow sandstones of Hoy really passed down into the flagstones 
as had been repeatedly stated by Sir R. Murcuison. For this purpose we sur- 
veyed with some care the northern end of the island. On the second visit we 
re-examined that locality and mapped the rest of the island. Having established 
an unconformable relation between the two formations, and having examined 
the various sections of a remarkable volcanic zone at the base of the yellow 
sandstones, we extended our explorations through the archipelago, crossing the 
Mainland in several directions, and then passing by steamer among the central 
and northern islands. We had not at our disposal the “six weeks,” which 
JAMESON found necessary for his traverse of Orkney, and we did not attempt to 
work out in detail the structure and succession of the strata. Fortunately, 
however, the coast sections are so continuous and clear, that one who has 
familiarised his eye with the characters of the flagstones in Caithness has no 
difficulty, even at a distance, in recognising the stratification and other features 
of that remarkable series of strata. We saw enough to enable us to follow the 
general arrangement of the rocks in the islands. The following narrative is 
drawn up from our observations :— 
1. Geological Structure and Sections.—Almost the whole of the Orkney — 
islands consist of flagstones and sandstones of the Caithness flag series. The 
only other formations present appear in the south-western part of this group. 
A small ridge of the underlying crystalline rocks rises to the surface at Strom- 
‘ness. In the island of Hoy a group of conical mountains with their underlying 
volcanic platform represents the upper division of the Old Red Sandstone. 
Here and there a few basalt dykes—far outlying portions, no doubt, of the great 
Tertiary series of the west of Scotland—cut through the flagstones with a pre- 
valent direction towards west or north-west. 
Each island may be looked upon as a low undulating plateau, truncated on 
one or more sides by a line of sea-cliff ranging in height up to 200 feet or 
upwards. In many places these cliffs rise directly out of the water, so that no 
tidal margin of débris occurs by which their base may be examined on foot. 
Like those of Caithness they are split by innumerable vertical or highly inclined 
joints, and have been eroded into long, deep, and narrow gullies or “goes,” 
often with great, square, detached columns of rock in front, and dim, resounding 
caves at the upper end. But where the coast shelves gently into the sea, 
the flagstones commonly appear upon the beach forming shore skerries, where 
each successive stratum may be followed. Hence there can hardly be any dif- 
il 

