The openings in rocks consist of joints and many other fractures, small spaces between the grains of rocks (pore space), and amygdaloidal and other openings characteristic of surface volcanic rocks. Many of these openings are capillary and sub-capillary in size. Most rocks, even dense igneous rocks, are porous in some degree, and certain rocks are porous in a very high degree. The voids in some surface materials may amount to 84 per cent of the total volume. In general the largest and most continuous openings are near the surface,--where rocks on the whole are more largely of the sedimentary type and are more fractured, disintegrated, and decomposed, than they are deep within the earth. The largest supplies of water are in the unconsolidated sediments. The water in igneous and other dense rocks is ordinarily in more limited quantity.
APPROXIMATE QUANTITY OF WATER WHICH WILL BE ABSORBED BY SOILS AND ROCKS[1]
------------------------------------------------------ _Volume of water asborbed _Material_ per 100 of material_ ------------------------------------------------------ Sandy soil[2] 45.4 Chalk soil[2] 49.5 Clay[2] 50-52.7 Loam[2] 45.1-60.1 Garden earth [2] 69.0 Coarse sand [2] 39.4 Peat subsoil[2] 84.0 Sand 30-40 Sandstone 5-20 Limestone and dolomite 1-8 Chalk 6-27 Granite 03.-.8 ------------------------------------------------------ 1: Mead, Daniel W., _Hydrology_: McGraw-Hill Book Co., New York, 1919, p. 393.
2: Woodward, H. B., _Geology of soils and substrata_: Edward Arnold, London, 1912.
Immediately at the surface, the openings of rocks may not be filled with water; but below the surface, at distances varying with climatic and topographic conditions, the water saturates the openings of the rocks and forms what is sometimes called the _zone of saturation_ or the _sea of underground water_. The top surface of this zone is called the _water table_, or the _ground-water level_. The space between the water table and the earth's surface is sometimes referred to as the _vadose zone_ or the _zone of weathering_, since it is the belt in which weathering processes are most active. The zone of weathering is not necessarily dry. Water from the surface enters and sinks through it and water also rises through it from below; it may contain suspended pockets of water surrounded by dry rocks; it is not continuously and fully saturated.