It needs but an infinitesimal increase in the quantity of oxygen in the air to produce a combustion which would melt all things. In pure oxygen, steel burns like a candle-wick. Nay, it is not necessary to increase the amount of oxygen in the air to produce terrible results. It has been shown[1] that, of our forty-five miles of atmosphere, one fifth, or a stratum of nine miles in thickness, is oxygen. A shock, or an electrical or other convulsion, which would even partially disarrange or decompose this combination, and send an increased quantity of oxygen, the heavier gas, to the earth, would wrap everything in flames. Or the same effects might follow from any great change in the constitution of the water of the world. Water is composed of eight parts of oxygen and one part of hydrogen. "The intensest heat by far ever yet produced by the blow-pipe is by the combustion of these two gases." And Dr. Robert Hare, of Philadelphia, found that the combination which produced the intensest heat was that in which the two gases were in the _precise proportions found in water_.[2] We may suppose that this vast heat, whether it came from the comet, or the increased action of the sun, preceded the fall of the _débris_ of the comet by a few minutes or a few hours. We have seen the surface-rocks [1. "Science and Genesis," p. 125. 2. Ibid., p. 127.] {p. 103} described as lustrous. The heat may not have been great enough to melt them--it may merely have softened them; but when the mixture of clay, gravel, striated rocks, and earth-sweepings fell and rested on them, they were at once hardened and almost baked; and thus we can account for the fact that the "till," which lies next to the rocks, is so hard and tough, compared with the rest of the Drift, that it is impossible to blast it, and exceedingly difficult even to pick it to pieces; it is more feared by workmen and contractors than any of the true rocks. Professor Hartt shows that there is evidence that some cause, prior to but closely connected with the Drift, did decompose the surface-rocks underneath the Drift to great depths, changing their chemical composition and appearance. Professor Hartt says: "In Brazil, and in the United States in the vicinity of New York city, the surface-rocks, under the Drift, are decomposed from a depth of a few inches to that of a hundred feet. The feldspar has _been converted into slate_, and the mica _has parted with its iron_."[1] Professor Hartt tries to account for this metamorphosis by supposing it to have been produced by warm rains! But why should there be warm rains at this particular period? And why, if warm rains occurred in all ages, were not all the earlier rocks similarly changed while they were at the surface? Heusser and Clarez suppose this decomposition of the rocks to be due to nitric acid. But where did the nitric acid come from?