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STONE: NATURAL AND ARTIFICIAL.
85
In sea-water, the weights (after deducting the weight of the volume of water,
which is the same for both) are 3456 Ibs. and 1728 Ibs.—a ratio of 2 to 1,
representing an increase of 33 per cent. As the exposure to wave-stroke is
the same in both cases, it is obvions that, when immersed, the stability of one
block has relatively increased from half as much again to twice that of the other.
The second point is hardness, or durability. A good stone in this respect
is one which is dense, compact, impervious, and free from all susceptibility to
disintegration. In maritime situations, stones are subjected to much wear and
friction—certainly more so than on land. The swell of the sea keeps those of
sma.11 size in a state of continuai agitation, rolling them over one another and
chafing them until they assume that smooth spherical or ellipsoidal form
which is so characteristic of pebbles along the beach. Moreover, in stormy
weather, shingle, shells, and gravel are taken up by the waves and dashed
with tremendous force against any surface upon which the waves happeu to
break. The efiect of continuai impact of this kind is to wear away even the
hardest masonry. Wave action is supplemented by that of the wind, which
blows sand in great volumes with the severity of a sand-blast. The cumulative
results of abrasion and attrition are to be observed on any rocky coast, where
towering cliffs stand honeycombed and fretted into fantastic shapes, while the
strand is strewn with the comminuted fragments of quondam boulders.
The chemical qualities of a stone are not perhaps of such striking
importance as its physical characteristics, but they are nevertheless deserving
of considération. The acidity and salinity of sea-water may, and often does,
bring about molecular changes in minerals containing soluble salts. Certain
compounds of lime are decomposed and softened by sea-water, and they also
give rise to the formation of other compounds which tend to destroy the
cohesion of the material of which they are ingredients, by producing cracks and
fissures. Caustic lime and caustic magnesia, which are to be found in inferior
and imperfectly made artificial stone or concrète—more rarely in natural
stone,—are causes of disintegration by reason of their expansion under
hydration, and also on account of their solubility. Still, on the whole, the
chemical aspect of the question assumes a secondary importance, because
those rocks which corne under the category of minerals available for marine
purposes, on account of their physical properties, are mostly, if not altogether,
free from unstable constituents. The only exception, perhaps, is granite,
which is a composition of three minerals—quartz, felspar, and mica, in a state
of physical, not chemical, incorporation. Of these, the quartz is durable beyond
cavil—it is practically indestructible; but certain varieties of felspar are
liable to decomposition, and the mica is always more or less easily disintegrated.
Nevertheless, granites, as a class, have gained a high reputation for strength
and permanence, and it is only in very inferior qualities that the imperfections
just mentioned manifest themselves, or where any appreciable deterioration is
produced by natural agencies.
The heaviest and most durable varieties of stone are, generally speaking,
those of igneous origin, such as basalts, granites, and traps, and metamorphic