These English "crystalline sandstones" were described by H. C. Sorby[35], who showed that the quartz deposited on the detrital grains was in optical continuity with that of the grains themselves. J. A. Phillips[36] regarded this quartz as crystallised out during the kaolinisation of felspars. The phenomena of laterisation, however, give us a further suggestion as to the origin of the secondary silica. It is now well known that tropical processes of weathering, with alternations of wet and dry seasons, allow alumina to be set free from combination with silica, "lateritic" crusts thus arising on a great variety of rocks. The felspars of a sandstone may, under such conditions, become laterised rather than kaolinised, aluminium hydrate being left, and the silica passing into solution and appearing again in certain layers as cementing quartz. The almost complete disappearance of silica from the more advanced laterites shows that it has been carried away elsewhere, and the cement of quartzite may thus be derived from rocks at a considerable distance. Just, however, as the destruction of siliceous sponge-spicules implies the formation of flint, so laterisation implies silicification as a complementary process. The fact that secondary quartz in quartzite often arises in the rock itself is shown by the frequency of quartz-veins in quartzites, while they are almost absent from associated slates or schists. Hence it appears that a removal of silica goes on at some points, leading to an infilling of all the cracks and interstices at another. It is clear, then, that sandstones, according to the mode in which they have been affected by percolating waters, may vary from the crumbling uncemented condition, known as _Sand-rock_, to that hardest and most resisting of rocks, quartzite. The permeability of sandstone is responsible for a wide variety of types. THE SAND-GRAINS OF SANDSTONE Sandstones are originally permeable by water, not because they possess a high percentage of pore-space, or "porosity," but because the pores between the grains are large. Water can thus move easily by gravitation through the mass. The capillary rise or spread of water is greatest in materials of very fine grain, though in these it may be extremely slow. For the most effective rise of water against gravity by capillary pull, a large proportion of particles about ·02 mm. in diameter should be present. Sand-grains, however, often measure ·5 mm. in diameter, and the fine mud or highly comminuted sand between the coarser matter is the cause of the spread of water through the mass when the supply comes from a subterranean water-table. Rain, however, is of course readily absorbed. It disappears so rapidly on some barren sandstone areas, coated as they are by loose sandy soils, that vegetation cannot make a start, even where water is supplied.