414 PROFESSOR GEIKIE ON THE 
fissures huge quadrangular buttresses, wedged off from the main cliff, rise in 
perilous independence above the surges of the northern sea. Where the 
dip of the rocks inclines seawards, and where, therefore, the strike-joints 
slope at high angles inland, the resultant cliff is often found to be a 
beetling, overhanging precipice. These features are well seen on the — 
western headlands of the Mainland, particularly on the Brough of Birsa. 
On the east side of Stronsa also they reappear in the noble precipice 
of Odin Ness. In these and the examples cited from Caithness, we see - 
how overhanging walls of rock owe their singular form, not to the under- 
mining of their base by the waves, but to the progress of weathering along 
their joints. Even, therefore, in so exposed a coast as that of the Orkney 
Islands, it is not so much the breakers (though their force is enormous), 
as the less conspicuous action of atmospheric agents, which cuts slice after 
slice from the edge of the land. 
C. The Shetland Islands. 
Beyond the furthest extremity of Orkney, at a distance of about 45 miles, 
rises the bold promontory of Sumburgh Head, the most southern point of Shet- 
land. Here again we encounter cliffs and skerries of the Old Red Sandstone, 
which so closely resemble those of Orkney and Caithness as at once to suggest 
that the whole form parts of one continuous area. By one who has spent 
some time among the latter localities, and has become familiar with the forms 
of cliff and goe, stack and cave, by which the flagstones are pierced, the out- 
lines of Sumburgh Head are readily recognised as belonging to the rocks of the 
northern type of Old Red Sandstone; and he naturally would anticipate another 
succession of flat cliff-girt islets like that which he has left behind him in — 
Orkney. But he soon discovers that here at last he seems to reach the limit of 
-the formation. He finds but a narrow margin of it on the south-east, and a 
still more limited tract on the south-west side of the Shetland group ; while the 
main mass of that group consists of crystalline rocks, which stretch northwards 
as if to connect themselves with those of Norway. 
The Old Red Sandstone of Shetland has been described by different 
observers ; but its geological position and its relation to the rest of the system 
in the north of Scotland have not been determined. Most of the accounts — 
which have been given of it have. been mineralogical.* In the beginning of 
1853 some fossil plants from the sandstones of Lerwick in Shetland, sent to the 
* See Jamuson, “ Mineralogy of the Scottish Isles” (1800), vol. ii. p. 186; Tram in “ Neill’s 
Tour in Orkney and Shetland,” 1806 ;, Fuemine, “ Memoirs of Wernerian Society,” vol. i. (1808), p. 
162; and in Satrerr’s “ Agriculture ae the Shetland Islands ” (1814), p. 120; Hippsrt, “ Descripata 
of the Shetland Islands” (1822), p. 157. 

