Free water exists in the openings in rocks where it is sometimes called _hygroscopic_ water. There is also a large amount of water combined molecularly with many of the minerals of rocks, in which form it is called _water of constitution_. This water is fixed in the rock so that it is not available for use, though some of the processes of rock alteration liberate it and contribute it to the free water. The immediate source of underground water, both free and combined, is mainly the surface or rain waters. A subordinate amount may come directly from igneous emanations or from destruction of certain hydrous minerals. Ultimately, as already indicated, even the surface water originates from such sources. 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.