distances. Their operations are more concealed from our view than those
of rivers, but extend over wider areas, and are therefore of more
geological importance.

_Waste of the British coasts._--_Shetland Islands_.--If we follow the
eastern and southern shores of the British islands, from our Ultima
Thule in Shetland to the Land's End in Cornwall, we shall find evidence
of a series of changes since the historical era, very illustrative of
the kind and degree of force exerted by tides and currents co-operating
with the waves of the sea. In this survey we shall have an opportunity
of tracing their joint power on islands, promontories, bays, and
estuaries; on bold, lofty cliffs, as well as on low shores; and on every
description of rock and soil, from granite to blown sand.

The northernmost group of the British islands, the Shetland, are
composed of a great variety of rocks, including granite, gneiss,
mica-slate, serpentine, greenstone, and many others, with some secondary
rocks, chiefly sandstone and conglomerate. These islands are exposed
continually to the uncontrolled violence of the Atlantic, for no land
intervenes between their western shores and America. The prevalence,
therefore, of strong westerly gales, causes the waves to be sometimes
driven with irresistible force upon the coast, while there is also a
current setting from the north. The spray of the sea aids the
decomposition of the rocks, and prepares them to be breached by the
mechanical force of the waves. Steep cliffs are hollowed out into deep
caves and lofty arches; and almost every promontory ends in a cluster of
rocks, imitating the forms of columns, pinnacles, and obelisks.

_Drifting of large masses of rock._--Modern observations show that the
reduction of continuous tracts to such insular masses is a process in
which nature is still actively engaged. "The isle of Stenness," says Dr.
Hibbert, "presents a scene of unequalled desolation. In stormy winters,
huge blocks of stones are overturned, or are removed from their native
beds, and hurried up a slight acclivity to a distance almost incredible.
In the winter of 1802, a tabular-shaped mass, eight feet two inches by
seven feet, and five feet one inch thick, was dislodged from its bed,
and removed to a distance of from eighty to ninety feet. I measured the
recent bed from which a block had been carried away the preceding winter
(A. D. 1818), and found it to be seventeen feet and a half by seven
feet