Much has been written by the observers on the "Challenger" and by others on the red clay of truly abyssal depths, which is attributed to the decay of wind-borne volcanic dust, and of igneous matter erupted on the sea-floor, rather than to any direct transport by water from the land. Clays may also accumulate on a land-surface from fine volcanic ash, which decomposes through the action of percolating waters. SLATE The relations between shale and _Slate_ are so obvious that slate may readily be regarded as a very well-compacted mud. The clayey material in it, like that of muds, may be ordinary detritus or of volcanic origin; its colours repeat those of shales. Its essential character, however, is the possession of a "cleavage," that is, of well-developed planes of fissility, which are often inclined to those of bedding. The bedding may be indicated by bands of different coarseness or constitution, and these may show crumpling due to pressure that has been exerted on the mass. The cleavage, however, may run right across these bands, and the rock, as a rule, splits far more cleanly along the cleavage-planes than a shale does along its planes of bedding. The early and historic observations on slaty cleavage have been excellently reviewed by A. Harker[47], who also provides an independent investigation. Reference may also be made to a later treatise by C. K. Leith[48], which contains numerous illustrations, and to a discussion by G. W. Lamplugh[49]. D. Sharpe and H. C. Sorby, between 1847 and 1853, developed the theory that rock-cleavage was due to compression in a direction perpendicular to the planes of cleavage and to expansion along them. As Harker points out, it is unlikely that the expansion balances the compression. The density of slate, about 2·7, is a good indication that the "porosity," or percentage of pore-space, has been reduced, while the mineral changes, soon to be referred to, are also in favour of greater density. C. Darwin[50] laid stress on the connexion between cleavage and the development of flaky minerals, such as micas, along the cleavage-planes, the structure ultimately passing into that known as "foliation" (see p. 145). H. C. Sorby urged that compression brings platy particles into parallel positions throughout the mass, so that the plates, which may consist of kaolin, mica, or chlorite, come to lie with their broad surfaces perpendicular to the direction of compression. At the same time, any constituents capable of deformation become compressed in this direction, become expanded in a direction perpendicular to it, and are themselves converted into lens-like forms or plates. T. Mellard Reade and P. Holland[51] have emphasised the part played by crystallisation at the close of the process of compression. They urge that the platy minerals, mica and chlorite, are produced during the alteration of the rock, and can spread with ease in directions perpendicular to that of compression; they thus give rise to