[98] It is probable that the porosity decreases in more than an arithmetic ratio, both because the deeper rocks are not of porous kinds, and because of the pressure which tends to close openings.
[99] Slichter (op. cit., p. 15) estimates that the ground-water is sufficient in amount to cover the earth’s surface to a depth of 3000 to 3500 feet. Earlier estimates gave still higher figures (see Delesse, Bull. Soc. Geol., France, Second Series, Vol. XIX, 1861–62, p. 64).
[100] Geikie. Text-book of Geology, 3d ed., p. 367.
[101] Ibid., p. 378.
[102] Prestwich, Q. J. Geol. Soc., Vol. XXVIII, p. lxvii.
[103] Reade. Liverpool Geol. Soc., 1876 and 1884.
[104] This is not true in the case of minerals, such as lime carbonate, dissolved under the influence of gases in solution in the water.
[105] Weed. The Formation of Hot Springs Deposits. Excursion to the Rocky Mountains. Compte Rendu. Fifth Session of the International Geological Congress, p. 360, and Ninth Ann. Rept. U. S. Geol. Surv., pp. 613–76. Also B. M. Davis, Science, Vol. VI, pp. 145–57, 1897.
[106] For a racy and interesting account of caverns see Shaler’s Aspects of the Earth.
[107] Russell has emphasized this point in 20th Ann. U. S. Geol. Surv., Pt. II, pp. 193–202, and Cross, 21st Ann. U. S. Geol. Surv., Part II, pp. 129–150.