In order to contribute enough heat to raise the surface temperature 1 deg.C., the temperature gradient from the interior of the earth to the surface would need to be ten times as great as now, for the rate of conduction varies directly with the gradient. If the gradient were ten times as great as now, the rocks at a depth of two and one-half miles would be so hot as to be almost liquid according to Barrell's[94] estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate that such high temperatures have not prevailed at such slight depths since the Proterozoic. Furthermore, the fact that the climate was cold enough to permit glaciation early in the Proterozoic era and at from one to three other times before the opening of the Paleozoic suggests that the rate of escape of heat was not rapid even in the first half of the earth's recorded history. Yet even if the general escape of heat has never been large since the beginning of the better-known part of geological history, it was presumably greater in early times than at present. If there actually has been an appreciable decrease in the amount of heat given out by the earth's interior, its effects would agree with the observed conditions of the geological record. It would help to explain the relative mildness of zonal, seasonal, and local contrasts of climate in early geological times, but it would not help to explain the long oscillations from era to era which appear to have been of much greater importance. Those oscillations, so far as we can yet judge, may have been due in part to solar changes, but in large measure they seem to be explained by variations in the extent, distribution, and altitude of the lands. Such variations appear to be the inevitable result of the earth's contraction. FOOTNOTES: [Footnote 70: Chas. Schuchert: The Earth's Changing Surface and Climate during Geologic Time; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 55.] [Footnote 71: Quoted by J. Cornet: Cours de Geologie, 1920, p. 330.] [Footnote 72: T. C. Chamberlin: The Order of Magnitude of the Shrinkage of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.] [Footnote 73: G. I. Taylor: Philosophical Transactions, A. 220, 1919, pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 308.] [Footnote 74: J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 309.] [Footnote 75: E. W. Brown: personal communication.] [Footnote 76: C. S. Slichter: The Rotational Period of a Heterogeneous Spheroid; in Contributions to the Fundamental Problems of Geology, by T. C. Chamberlin, _et al._, Carnegie Inst. of Wash., No. 107, 1909.] [Footnote 77: E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.] [Footnote 78: Chas. Schuchert: The Earth's Changing Surface and Climate; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.]