Page:Popular Science Monthly Volume 34.djvu/678

ashes—no river, save of molten fire. Now is ending a long history with which the uniformitarian must not reckon—of a time when many compounds now existing were not dissolved but dissociated, for combination under that environment was impossible. Yet there was still law and still order—nay, the present law and order may be said even then to have had a potential existence; nevertheless, to the uniformitarian gnome, had such there been, every new combination of elements would have been a new shock to his faith, a new miracle in the earth's history. But at the times mentioned above, though oxygen and hydrogen could combine, water could not yet rest upon the ruddy crust of the globe. What does that mean? This, that assuming the water of the ocean equivalent to a spherical shell of the earth's radius and two miles thick, the very lava-stream would consolidate under a pressure of about 310 atmospheres, equivalent to nearly 4,000 feet of average rock. Let us pass on to a time, which, according to Sir W. Thomson, would rather quickly arrive, when the surface of the crust had cooled by radiation to its present temperature. Let us merely, for illustration, take a surface temperature of 50° F. (nearly that of London), and assume that the present rise of crust temperature is 1° F. for every fifty feet of descent, which is rather too rapid. If so, 213° F. is reached at 8,100 feet, and 250° F. at 10,000 feet. Though the latter temperature is far from high, yet we should expect that, under such a pressure, chemical changes would occur with much more facility than at the surface. But many Palæozoic, or even later rock-masses, can now be examined which at a former period of their history have been buried beneath at least 10,000 feet of sediment, yet the alteration of their constituents has been small; only the more unstable minerals have been somewhat modified, the more refractory are unaffected. But for a limited period after the consistentior status, the increase of crust temperature in descending would be far more rapid; when one twenty-fifth of the whole period from that epoch to the present had elapsed—and this is no inconsiderable fraction—the rate of increase would be one degree for every ten feet of descent. Suppose, for the sake of comparison, the surface temperature as before, the boiling-point of water would be reached at 1,620 feet, and at 10,000 feet, instead of a temperature of 250° F., we should have one of 1,050° F. But, at the latter temperature, many rock-masses would not be perfectly solid. According to Sorby, the steam cavities in the Ponza trachyte must have formed, and thus the rock have been still plastic at so low a temperature as 680° F. At this period, then, the end of the fourth year of the geological century, structural changes in igneous and chemical changes in sedimentary rocks must have taken place with greater facility than in any much later period in the world's history. Then a temperature of