The Pacific "rim" is a girdle surrounding the deep ocean basin of the Pacific; if we neglect the curvature of the earth, it may be likened to the raised rim of a tray. Over vast areas of the "basin" the floor is comparatively smooth?but not quite smooth, as there is an unexplained small relief of "valleys, ridges, and troughs" (Menard, 1956) that has recently been discovered and is ubiquitous except where obliterated by accumulation of lava and of the sediment forming "archipelagic aprons" (Menard). Of a much larger order, however, are scattered mounds of volcanic origin which are of huge size, though dwarfed by the vast dimensions of the surrounding basin. Some of the volcanoes that originated on the sea floor have been built up to form existing islands, but others fail to reach within several hundred fathoms of the surface of the sea, either having become the foundations of atolls or remaining sub? merged as seamounts. Away from these (and not far from them) the water is very deep, though the maximum depths, up to 6 miles, are confined to the narrow but elongated ocean deeps. In contrast with some monotony on the basin floor, relief is varied in many parts of the rim, consisting of mountain chains, though in other places are curving festoons of islands, not all of them high. The rim does not coincide in position with the geo? graphical boundary of the ocean; rather is its position definable in geophysical and geological terms, so that the rim concept is as much geological as geographical. In the western Pacific it is far out in the Pacific Ocean, though necessarily in such a place it is discontinuous, consisting of islands. Behind "the rim, to the west, is a shelf-like belt of marginal seas that are in general shallower than the basin. In terms of the classic sial and sima, the floor of the basin consists of sima which is almost bare,* its only cover, as indicated by geophysical investigation supported by seismic evidence, being a thin layer which must consist for the most part of abyssal sediment; and the Mohorovi?ic discontinuity is at a depth rarely greater than 6 miles below sea level. Over the floors of the marginal seas outside the rim, however, there is some sial, though it is a layer very much thinner than that which forms the continents (Gutenberg, 1951). A remark of Officer et al. (1957) made with regard to the contrasting floors of the Atlantic Ocean and Caribbean Sea is applicable to those of the Pacific basin and the western seas. Unless they were different "it would be difficult to understand how an island are and deep-sea trench might develop between them."