As agents of transportation, ocean-currents are scarcely more important than as agents of corrasion, for they transport only what they erode, if the life which inhabits them be left out of consideration. This phase of their work has probably been exaggerated through a confusion of transporting energy and actual transportation. Ocean-currents which do not touch bottom roll no sediment and carry only what may be held in suspension. A river’s power of transporting sediment in suspension is due largely to the cross-currents occasioned by the unevenness of its resistant bottom (p. 117). If a particle of mud in suspension in a river drops to the bottom, as it frequently does, it may be picked up again and carried forward. If, on the other hand, a particle in suspension in an ocean-current once escapes the moving water by settling through it, the current which does not drag bottom has no chance to pick it up again. Very fine sediment may be carried by an ocean-current far beyond the point where it was acquired, but currents which do not touch bottom are rarely strong enough to hold any but the finest material for any considerable length of time. As transporters of sediment, therefore, ocean-currents are at a great disadvantage as compared with rivers. How readily particles of extreme fineness may be kept in suspension, and how little agitation is necessary to keep them from sinking, is shown by the experiments of Sorby, who showed that while a sand grain ¹⁄₁₀₀ of an inch in diameter will settle one foot per second in still water, fine particles of clay require days to sink through the same distance. The Challenger found fine sediment derived from the land 400 miles from the coast of Africa, and that not opposite the debouchure of any large river. Sediment settles more readily in salt water than in fresh, despite the fact that the former is heavier. This is presumably because the salt diminishes the cohesion of the water. Deposition by ocean-currents is limited by their transportation. Only where they erode their bottoms do they gather coarse materials, and only in the lee of such places are their deposits coarse. Since the material which they carry is generally fine, it is widely distributed before deposition. Ocean-currents have little influence on the configuration of coast-lines. DEPOSITS ON THE OCEAN-BED. Something has already been said concerning the sediments which accumulate in the shallow waters along shores; but the area of marine sedimentation is as extensive as the ocean itself, and the deposits must now be reviewed from another point of view.