=The zone of cementation.= With increasing depth subterranean water becomes more and more sluggish in its movements and more and more highly charged with minerals dissolved from the rocks above. At such depths it deposits these minerals in the pores of rocks, cementing their grains together, and in crevices and fissures, forming mineral veins. Thus below the zone of solution where the work of water is to dissolve, lies the zone of cementation where its work is chemical deposit. A part of the invisible load of waste is thus transferred from rocks near the surface to those at greater depths. As the land surface is gradually lowered by weathering and the work of rain and streams, rocks which have lain deep within the zone of cementation are brought within the zone of solution. Thus there are exposed to view limestones, whose cracks were filled with calcite (crystallized carbonate of lime), with quartz or other minerals, and sandstones whose grains were well cemented many feet below the surface. =Cavity filling.= Small cavities in the rocks are often found more or less completely filled with minerals deposited from solution by water in its constant circulation underground. The process may be illustrated by the deposit of salt crystals in a cup of evaporating brine, but in the latter instance the solution is not renewed as in the case of cavities in the rocks. A cavity thus lined with inward-pointing crystals is called a _geode_. =Concretions.= Ground water seeping through the pores of rocks may gather minerals disseminated throughout them into nodular masses called concretions. Thus silica disseminated through limestone is gathered into nodules of flint. While geodes grow from the outside inwards, concretions grow outwards from the center. Nor are they formed in already existing cavities as are geodes. In soft clays concretions may, as they grow, press the clay aside. In many other rocks concretions are made by the process of _replacement_. Molecule by molecule the rock is removed and the mineral of the concretion substituted in its place. The concretion may in this way preserve intact the lamination lines or other structures of the rock (Fig. 34). Clays and shales often contain concretions of lime carbonate, of iron carbonate, or of iron sulphide. Some fossil, such as a leaf or shell, frequently forms the nucleus around which the concretion grows. Why are building stones more easily worked when "green" than after their quarry water has dried out? [Illustration: Fig. 34. Concretions in Sandstone, Wyoming]