[97] _Tenby_, p. 169, Plates VII. and VIII.
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.