DESTRUCTION OF STONE.
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breakwater had to be replaced by granite blocks owing to the ravages of
the mollusc. Apart from this, the growing popularity and the ready
adaptability of concrete have caused it to largely supersede natural rock
for dock construction and harbour works.
Destruction of Stone.—The softer kinds of stone will frequently wear
away under continued attrition and the chemical action of an unsuitable
atmospheric environment, but the destructive agencies most in evidence,
in regard to the more adamantine varieties used in dock work, are living
organisms.
The Pholas dactylus is a mollusc, living in seawater, which bores into
limestone, shale, sandstone, and timber, but does not attack granite. It
is a small animal, with a maximum length of about 5 inches, but one
which is quite capable of doing extensive mischief by boring its holes
in close proximity to each otber, causing the ultimate collapse of the
masonry....
The Samcava is another mollusc known to bore into limestone to a
depth of 6 inches. It has manifested its presence at Plymouth, Folke-
stone, and elsewhere.
There is apparently no remedy for the ravages of these marine borers,
except the substitution of some other kind of material for the stone
attacked.