When once raised beyond the level of the highest tide, the eroded
surface appears to have a permanency which defies the action of the
elements for an undefinable period; for it seems liable to little
change. It is probably comparatively unalterable, or alterable slowly,
beneath the level of the lowest tide. But between tide-marks, the
perpetual change from wetness to dryness and back again, and the
incessant wash of the waves, which frequently beat and dash upon the
eroded surface with immense violence, are continually grinding down
the projecting points and thin walls of stone, and thus creating a new
surface, to be bored afresh by new generations of Mollusca.
It has seemed to me that these burrows have played and are playing
an important part in the formation of the numberless rocky basins
which we call tide-pools, and in which we marine naturalists so much
delight. Let us look at the process. About half-tide level there is a
mass of bored rock, from whose burrows the tenants are dying out for
want of sufficiently long water-covering. A heavy sea is breaking over
it, which has snapped off the thin partition beneath two contiguous
burrows, breaking it into several sharply angular bits, which fall into
the hole. The whirling and eddying of the waves rattle and roll these
fragments round and round day after day, week after week, till at last
they are ground to nothing: but an equal effect has been produced on
the hollow which held them; its cavity has been widened and deepened
by the same grinding action. By-and-by a pebble is rolled in, and being
almost large enough to fill the cavity, it does not readily wash out,
but grinds round and round with the motion of the sea. So the process
goes on, perhaps for centuries, perhaps with long intervals of almost
sameness; every stone that is washed-in enlarging the work; while, when
once the hollow has become only ever so little larger or deeper than
those which surround it, the pebbles will have an increasing tendency
to roll in and to stay there. So, at length, the basin is formed,
tiny at first;--I know scores not so big as a slop-bowl, which yet
have their furniture of elegant little sea-weeds, green and purple,
and their tenants of worms, and shrimps, and polyps;--but destined
by-and-by to become noble reservoirs in which man may pleasantly bathe,
and in which little fishes play and shoot to and fro, and hide under
the umbrageous fronds of the oar-weed and tangle that droop gracefully
into the ample cavity.