When the detritus, still somewhat mixed, reaches a sea-shore, wave-action is rapidly effective. Before the continual wash and pounding of the water, any residual clay, and the finely comminuted portion of the quartz, are carried down the coastal slope. The colour of the sea after storms is sufficient evidence of the work that it performs. Beaches, then, arrive at a great similarity of type. The inviting yellow sands, formed of comparatively coarse material, occur alike off shores formed of chalk, slate, granite, or boulder-clay. [Illustration: Fig. 5. Sand Developing from Sandstone, in semi-arid climate. Near Laingsburg, Cape of Good Hope.] From the beginning of sedimentation, sands have thus tended to accumulate, and to become cemented into sandstones. These rocks, in turn uplifted and exposed, have yielded other sandstones. Since coarse sand does not travel far from the region where it is washed out of the parent rock, a thick mass of sandstone extending over many square miles may waste away, and yet become perpetuated in the district. Sandiness thus begets sandiness, and the physical conditions due to the presence of sandstone may prevail through long geological epochs (Fig. 5). Of course, a submergence beneath the sea may change all this in a brief time; but wrinklings of the crust, raising the sandstones into severer atmospheric levels, may only accelerate their decay and render the surrounding lands more sandy. THE CEMENTING OF SANDS The cement of sandstones is very varied. On our modern coasts, springs draining from a limestone land, or even running through banks of broken shells, will deposit calcite in the interstices of the beach, until slabs and shelves of conglomerate and sandstone arise in defiance of the waves. On coasts where calcium bicarbonate is abundant, it may be precipitated by any cause that diminishes its solvent. Mere evaporation, and the escape of carbon dioxide from the water as it is scattered into spray, lead to the deposition of a cement between the grains of sand. As Linck[6] shows, calcite is thus laid down in temperate waters, while aragonite forms fibrous crystals between the detrital fragments on the flanks of tropic isles. Aragonite may also arise from the action of ammonium carbonate or sodium carbonate on calcium sulphate or calcium chloride in sea-water. Sands thus become cemented by one or other form of calcium carbonate. They include, moreover, calcareous algæ, foraminifera, and fragments of coral and sea-shells. Fossil shells are usually represented in older sandstones by mere external and internal moulds. The texture of the rock allows of their being dissolved in percolating waters, while in clays belonging to the same geological series they may be exquisitely preserved.