t in Roeness, Dr. Hibbert, writing in 1822, enumerates other
ravages of the ocean. "A mass of rock, the average dimensions of which
may perhaps be rated at twelve or thirteen feet square, and four and a
half or five in thickness, was first moved from its bed, about fifty
years ago, to a distance of thirty feet, and has since been twice turned
over."

_Passage forced by the sea through porphyritic rocks._--"But the most
sublime scene is where a mural pile of porphyry, escaping the process
of disintegration that is devastating the coast, appears to have been
left as a sort of rampart against the inroads of the ocean;--the
Atlantic, when provoked by wintry gales, batters against it with all the
force of real artillery--the waves having, in their repeated assaults,
forced themselves an entrance. This breach, named the Grind of the Navir
(fig. 28), is widened every winter by the overwhelming surge that,
finding a passage through it, separates large stones from its sides, and
forces them to a distance of no less than 180 feet. In two or three
spots, the fragments which have been detached are brought together in
immense heaps, that appear as an accumulation of cubical masses, the
product of some quarry."[393]

[Illustration: Fig. 28.

Grind of the Navir--passage forced by the sea through rocks of hard
porphyry.]

It is evident from this example, that although the greater
indestructibility of some rocks may enable them to withstand, for a
longer time, the action of the elements, yet they cannot permanently
resist. There are localities in Shetland, in which rocks of almost every
variety of mineral composition are suffering disintegration; thus the
sea makes great inroads on the clay slate of Fitfel Head, on the
serpentine of the Vord Hill in Fetlar, and on the mica-schist of the Bay
of Triesta, on the east coast of the same island, which decomposes into
angular blocks. The quartz rock on the east of Walls, and the gneiss and
mica-schist of Garthness, suffer the same fate.

_Destruction of islands._--Such devastation cannot be incessantly
committed for thousands of years without dividing islands, until they
become at last mere clusters of rocks, the last shreds of masses once
continuous. To this state many appear to have been reduced, and
innumerable fantastic forms are assumed by rocks adjoining these islands
to which the name of Drongs is applied, as it is to those of similar
shape in Feroe.

[Illustration: Fig. 29.