406 PROFESSOR GEIKIE ON THE 
arrangement of these rocks. It will be seen that they are exposed between 
tide-marks, and that they lie near the centre of the trough of sandstones. A 
dyke of diabase cuts through the sandstones, while immediately to the east lies 
a well-marked neck of volcanic agglomerate, about 300 feet in diameter. This 
rock consists of a dull greenish diabase paste, stuck full of blocks of diabase, 
red sandstone, flagstone, limestone, black cleavable augite,* gneiss, &c. Some 
diabase blocks measure three feet or more across. On the west side of the 
neck some irregular veins, or perhaps huge included amorphous masses of 
diabase and hardened agglomerate occur. The sandstone round the margin of 
the neck is perceptibly hardened and jointed. 
It is impossible to regard without some interest this little volcanic pipe 
placed alone almost on the extreme north-eastern point of Scotland. When it 
was discovered by Mr B. N. PeaAcu and myself, we knew of no other volcanic 
mass associated with the Old Red Sandstone on the north side of the 
Grampian mountains. Not a single dyke or bed had up to that time appeared 
to us among the wonderfully continuous coast sections of Caithness. We 
could not but suspect, however, that this ancient volcano of John o’ Groat’s 
might be one of a series, which, as we had failed to find any other of its members” 
to the south, might hopefully be sought for among the Orkney Islands. Its | 
precise geological date could not of course be fixed from the evidence of the 
neck itself. In lithological character, it quite resembles some of the agglome- 
rates associated with the Old Red Sandstones and Lower Carboniferous rocks 
of central Scotland, and we were inclined to regard it as probably an outlier of 
some volcanic series associated with the Upper Old Red Sandstones of the 
north of Scotland. This conjecture was soon after verified by the discovery of 
a much more abundant and complete volcanic series at the base of the Upper 
Old Red Sandstone of the island of Hoy. 
B. The Orkney Islands. 
From the northern headland of Caithness the eye looks across the Pentland 
Firth to the long, low, featureless outline of Orkney, which seems to be rather a | 
single detached area of the tableland of Caithness than a series of independent 
islands. To the westward, indeed, the low land rises into the rounded hills of | 
Hoy, which descend in lofty lines of yellow cliff into the sea; but with this | 
exception, neither as seen from a distance, nor, when actually visited, is| 
there any rough or hilly ground to be met with in Orkney. The scenery of} 
one island is almost entirely the counterpart of that of its neighbours. Each 
forms a little table-land, sometimes entirely girded by cliffs, but usually sloping} 
* T supplied Dr Hepp ie with a piece of this remarkable augite for analysis. His determination | 
of its composition and my description of its. occurrence will be found in his paper published in this} — 
volume. 
ae 

