What eager pursuer of marine animals has not gloated over a rock-pool?
On all our rocky coasts we find them more or less developed; but it
is on these south-western shores, where the compact limestone juts
out into promontories, that we find them in perfection. The burrowing
mollusca specially favour the limestone; the _Saxicava_, I think, lives
in no other medium; and it is to the operation of this coarse ugly
little shell-fish that this rock is indebted for the honeycomb-like
excavation which has eroded its surface. Below a few inches this
erosion does not extend, for the _Saxicava_ is but a small animal, and
its siphons must reach the orifice of its burrow; therefore it never
goes deeper into the stone than will allow it comfortably to bathe its
red nose in the free water, though it is not at all particular about
the angle to the surface at which it bores. The myriads and myriads
of these auger-holes that have been bored remain, though the feeble
animals perish generation after generation; each new-born shell-fish
makes a new bore for itself, never appropriating one ready-made, and so
there is a perpetual excavation of the living rock with these shallow
auger-holes, always of the same width, or nearly; about half-an-inch.
The result is what we see; that the surface of the rock knows no such
thing as a plane surface, but a surface covered with smooth borings,
running in all directions, so as continually to break in on one
another; and that so close together, that the interspaces form narrow
knife-edges, and sharp angles and projecting points. A particularly
interesting circumstance is, that this honeycombed condition is
characteristic not only of that level of the rock which is covered by
the sea for some portion of every tide, but of that part, to a certain
height, which is never covered at all. The Mollusca, it is true, cannot
live wholly deprived of sea-water, and, in fact, there are none in
this ever-dry portion, though the burrows by thousands testify that
they were there once. We must infer that the coast has been generally
elevated; perhaps by slow and imperceptible degrees, by an operation
still proceeding but unappreciated; perhaps by some sudden convulsion
which took place at a remote era, unrecorded and forgotten.