I have frequently walked the shore, and observed the colour of the waves, after what is termed a “ground swell,” which had lasted, perhaps, for thirty or forty hours. The cerulean hue is then gone, and to it has succeeded, in certain localities, an opaque chalky tinge, showing that the water is now heavily charged with lime. Also fragments of shells rolled together are united with heavy masses of sand, and sometimes of broken pumice-stone, and a kind of rough marl is rapidly formed, and left on the beach. After a gale, and succeeding “swell,” I have met with these imperfect boulders, varying in size from that of a man’s fist to some larger than his head. At the same time, any low ranges of littoral rocks become crusted over with the superabundant lime, being more than the waves will long hold in solution; and a coating is thus given to such rocks which is sometimes as hard as is the native limestone itself, a few weeks’ exposure to the sun and air sufficing to effect this. Some fourteen years ago, I had an opportunity, when in Sicily, of examining a portion of the coast between Messina and Catania, and I regret that I did not avail myself of it more heartily. But I have seen M. Quatrefage’s book on this subject, and his observations, most carefully and laboriously conducted, may almost be said to close that part of the subject, as far as any prospect of eliciting fresh information is concerned. I think he measured some of the long reefs, and the evident increase _by incrustation_ extended for many miles of the coast-line, and was of considerable thickness. I have observed the same “masonic” phenomenon off the coast of St. Andrews, in Fifeshire, when swimming out among the weedy rocks, and afterwards climbing to the shore. I then thought it was the work of marine insects, as I had heard of molluscs building causeways of tubes of limestone, but I incline now to think it was the sea doing it, as they say, at _first-hand_. The sea, however, does a great deal in the same line at second-hand, by means, chiefly, of two species of zoanthoid polyps. Of these little creatures, one kind constructs the bases of coral islands, and another the summits (at the least) of the madrepore reefs. It is impossible to doubt that by labours so patiently carried on, and so widely diffused, some beneficent purpose is aimed at and attained by Providence. Perhaps they operate, finally, to warn the shipping of adventurous merchants from entering certain dangerous bays and straits; and if the humble madrepore has got a bad name through this, as though he had made the danger, he certainly suffers unjustly, for what real difference, as to ultimate and assured safety, can a few inches more of water in such places make? Far better to raise an impassable bar across the way at once, and proclaim, “no thoroughfare!”