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. 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? DEPOSITS OF GROUND WATER IN ARID REGIONS. In arid lands where ground water is drawn by capillarity to the surface and there evaporates, it leaves as surface incrustations the minerals held in solution. White limy incrustations of this nature cover considerable tracts in northern Mexico. Evaporating beneath the surface, ground water may deposit a limy cement in beds of loose sand and gravel. Such firmly cemented layers are not uncommon in western Kansas and Nebraska, where they are known as "mortar beds." THERMAL SPRINGS. While the lower limit of surface drainage is sea level, subterranean water circulates much below that depth, and is brought again to the surface by hydrostatic pressure. In many instances springs have a higher temperature than the average annual temperature of the region, and are then known as thermal springs. In regions of present or recent volcanic activity, such as the Yellowstone National Park, we may believe that the heat of thermal springs is derived from uncooled lavas, perhaps not far below the surface. But when hot springs occur at a distance of hundreds of miles from any volcano, as in the case of the hot springs of Bath, England, it is probable that their waters have risen from the heated rocks of the earth's interior. The springs of Bath have a temperature of 120 degrees F., 70 degrees above the average annual temperature of the place. If we assume that the rate of increase in the earth's internal heat is here the average rate, 1 degree F. to every sixty feet of descent, we may conclude that the springs of Bath rise from at least a depth of forty-two hundred feet. Water may descend to depths from which it can never be brought back by hydrostatic pressure. It is absorbed by highly heated rocks deep below the surface. From time to time some of this deep- seated water may be returned to open air in the steam of volcanic eruptions.