sand, rock material occurring in the form of loose, rounded or angular grains, varying in size from .06 mm to 2 mm in diameter, the particles being smaller than those of gravel and larger than those of silt or clay. Sand is formed as a result of the weathering and decomposition of igneous, sedimentary, or metamorphic rocks. Its most abundant mineral constituent is silica, usually in the form of quartz, and many deposits are composed almost exclusively of quartz grains. Many other minerals, however, are often present in small quantities, e.g., the amphiboles, the pyroxenes, olivine, glauconite, clay, the feldspars, the micas, iron compounds, zircon, garnet, tourmaline, titanite, corundum, and topaz. Some sands—e.g., coral sands, shell sands, and foraminiferal sands—are organic in origin. Sand grains may be rounded or more or less angular, and differences in shape and size account chiefly for differences in such important properties as porosity (proportion of interstices to the total mass), permeability to gases and liquids, and viscosity, or resistance to flow. Permeability and viscosity are also affected by the proportion of clayey matter present. The chief agents in accumulating sands into deposits are winds, rivers, waves, and glaciers; sand deposits are classified according to origin as fluviatile, lacustrine, glacial, marine, and eolian. The most extensive superficial deposits are seen in the desert and on beaches. The surface of a sand deposit may be level or very gently sloping, or the sand may be gathered by wind action into ridges called dunes. Sandstone and quartzite rocks are indurated masses of sand, and sand deposits are sometimes formed by the weathering of sandstone and quartzite formations. Sand is used extensively in the manufacture of bricks, mortar, cement, concrete, plasters, paving materials, and refractory materials. It is also used in the metallurgical industry, in the filtration of water, in pottery making, in glassmaking, in the manufacture of explosives, and as an abrasive. Other industrial uses are numerous. Although soils entirely composed of sand are too dry and too lacking in nourishment for the growth of plants, a soil that is to some extent sandy (a "light" soil) is favorable to certain types of agriculture and horticulture, as it permits the free movement of air in the soil, offers less resistance than a clay soil to growing roots, improves drainage, and increases ease of cultivation. Sand to which nutrient solutions have been added is often used in soilless gardening.