at a speed which varies from 6 to 12 m. an hour. At the meeting
of the western and eastern currents the waves at times rise into the air
like a waterspout, but the current does not always nor everywhere flow at a
uniform rate, being broken up at places into eddies as perilous as itself.
The breakers caused by the sunken reefs off Duncansbay Head create the
Bores of Duncansbay, and eddies off St John's Point are the origin of the
Merry Men of Mey, while off the island of Stroma occurs the whirlpool of
the Swalchie, and off the Orcadian Swona is the vortex of the Wells of
Swona. Nevertheless, as the most direct road from Scandinavian ports to the
Atlantic the Firth is used by at least 5000 vessels every year. In the
eastern entrance to the Firth lies the group of islands known as the
Pentland Skerries. They are four in number--Muckle Skerry, Little Skerry,
Clettack Skerry and Louther Skerry--and the nearest is 41/2 m. from the
mainland. On Muckle Skerry, the largest (1/2 m. by 1/3 m.), stands a
lighthouse with twin towers, 100 ft. apart. The island of Stroma, 11/2 m.
from the mainland (pop. 375), belongs to Caithness and is situated in the
parish of Canisbay. It is 21/4 m. long by 11/4 m. broad. In 1862 a remarkable
tide climbed the cliffs (200 ft.) and swept across the island.

_Geology._--Along the western margin of the county from Reay on the north
coast to the Scaraben Hills there is a narrow belt of country which is
occupied by metamorphic rocks of the types found in the east of Sutherland.
They consist chiefly of granulitic quartzose schists and felspathic
gneisses, permeated in places by strings and veins of pegmatite. On the
Scaraben Hills there is a prominent development of quartz-schists the age
of which is still uncertain. These rocks are traversed by a mass of granite
sometimes foliated, trending north and south, which is traceable from Reay
southwards by Aultnabreac station to Kinbrace and Strath Helmsdale in
Sutherland. Excellent sections of this rock, showing segregation veins, are
exposed in the railway cuttings between Aultnabreac and Forsinard. A rock
of special interest described by Professor Judd occurs on Achvarasdale
Moor, near Loch Scye, and hence named Scyelite. It forms a small isolated
boss, its relations to the surrounding rocks not being apparent. Under the
microscope, the rock consists of biotite, hornblende, serpentinous
pseudo-morphs after olivine and possibly after enstatite and magnetite, and