"They have been usually attributed to the action of waves; but the mechanical work of the ocean is mostly confined to its shores and soundings, where alone material exists in quantity within reach of the waves and currents.[2] . . . The eroding action is greatest for a short distance above the height of half-tide, and, except in violent storms, it is almost null below low-tide."[3] But if any one will examine a sea-beach he will see, not a vast mass of pebbles perpetually rolling and grinding each other, but an expanse of sand. And this is to be expected; for as soon as a part of the pebbles is, by the attrition of the waves, reduced to sand, the sand packs around the stones and arrests their further waste. To form such a mass of gravel as is found in the Drift we [1. "Edinburgh Review," October, 1874, p. 202. 2. Dana's "Text Book," p. 286. 3. Ibid., p. 287.] {p. 79} must conceive of some way whereby, as soon as the sand is formed, it is removed from the stones while the work of attrition goes on. This process we can conceive of in a comet, if the finer _detritus_ is constantly carried back and arranged in the order of the size of its particles. To illustrate my meaning: let one place any hard substance, consisting of large fragments, in a mortar, and proceed to reduce it with a pestle to a fine powder. The work proceeds rapidly at first, until a portion of the material is triturated; you then find that the pulverized part has packed around and protected the larger fragments, and the work is brought to a stand-still. You have to remove the finer material if you would crush the pieces that remain. The sea does not separate the sand from the gravel; it places all together at elevations where the waves can not reach them: "Waves or shallow soundings have some transporting power; and, as they always move toward the land, their action is landward. They thus beat back, little by little, any _detritus_ in the waters, preventing that loss to continents or islands which would take place if it were carried out to sea."[1] The pebbles and gravel are soon driven by the waves up the shore, and beyond the reach of further wear;[2] and "_the rivers carry only silt to the ocean_."[3] The brooks and rivers produce much more gravel than the sea-shore: "The _detritus_ brought down by rivers is vastly greater in quantity than the stones, sand, or clay produced by the wear of the coasts."[4] [1. Dana's "Text Book," p. 288. 2. Ibid., p. 291. 3. Ibid., p. 302. 4. Ibid., p. 290.] {p. 80} But it would be absurd to suppose that the beds of rivers could have furnished the immeasurable volumes of gravel found over a great part of the world in the drift-deposits. And the drift-gravel is different from the gravel of the sea or rivers. Geikie says, speaking of the "till":