334 ON THE MOVEMENTS OF THE EARTH'S CRUST. 



the sea will Lave flowed towards the poles ; but wbeu the teusion be- 

 comes so great that the solid crust bursts, the equatorial regions will 

 sink in and the sea will flow again towards the equator.* 



In the Philosophical Transactions for 1879, parts i and il, Prof. G. 

 Darwin has published a memoir the results of which are briefly as fol- 

 lows. He assumes that the earth possesses a small degree of plasticity, 

 and calculates the internal friction which the tidal action of the moon 

 and sun produce in such a body. He finds that both the sidereal day 

 and the month have become much longer, that the distance of the moon 

 has increased, that the obliquity of the eclii^tic has diminished, and that 

 a great part of the internal heat is developed by the internal friction. 

 Forty-six million three hundred thousand years ago, according to his 

 calculation, the sidereal day was 15'' 30"", and the moon's distance 46.8 

 terrestrial radii (against 60.4 at present). But 56,180,000 years ago the 

 sidereal day was only 6'^ 45'" long, the moon's distance only 9 terrestrial 

 radii, and the month only 1.58 day (one-seventeenth of its present 

 amount). The interior heat produced by friction in 57,000,000 years, 

 if applied at once, would suffice to heat the whole earth 1700'^ Fahr. t 

 He concludes that the compression has constantly diminished : " the 

 polar regions must have been ever rising, and the equatorial ones falling 

 though as the ocean followed these changes they might quite well have 

 left no geological traces.f The tides must have been very much more 

 frequent and larger, and accordingly the rate of oceanic denudation much 

 accelerated. The more rapid alternation of day and night [57,000,000 

 years ago, according to Darwin, the year had 1,300 days] would proba- 

 bly lead to more sudden and violent storms ; and the increased rota- 

 tion of the earth would augment the violence of the trade-winds, which, 

 in their turn, would aflect oceanic currents. § 



Tresca {Gomptes Bendus, 1864, p. 754; 1867, p. 802, etc.) has shown 



* Similar opinions are expressed by Dr. E. Reyer (" Die Bewegung im Festen," in 

 Jahrl). K. K. Geol. Reichn. Wien, 1880, vol. xxx, pp. 543 et seq.). W. B. Taylor, in a 

 memoir " On the Crumpling of the Earth's Crust " {Amer. Journ. ScL, 1885, ser 3, vol. 

 xxx, pp. 249 et seq.), expresses himself against the theory of the earth's contraction, 

 and thinks that the lengthening of the sidereal day is the cause of the changes in the 

 crust. A. Winchell, in a memoir on the " Sources of Trend and Crustal Surplusage " 

 (Amer. Journ, Sci. I. e, p. 417), endeavors to show that the diminishing centrifugal force 

 has produced foldings in a north and south direction. J. E. Todd, in a papier entitled 

 "Geological Effects of a "Varying Rotation of the Earth" (Amer. Naturalist, 1883, 

 vol. XVII, pp. 15 et seq.), first enumerates the various forces which may act in acceler- 

 ating and retarding the axial rotation. He assumes that the axial rotation decreases 

 and increases abruptly, that it acts first upon the sea and afterwards upon the solid 

 crust, and that for this reason the sea rises and sinks abruptly in relation to the land. 



tThis beat, produced by the internal friction, must contribute considerably to 

 diminish the secular refrigeration. Lapparent has not taken account of this in the 

 above-cited memoir on the contraction and cooling of the earth. 



tin a subsequent article, however, Darwin supposes that the coast-lines will shift 

 in consequence of the lengthening of the sidereal day (Nature, Sept. 2, 1886, p. 422;. 



§ These numerical values make no claim to represent the actual values; they are 

 merely the niaximnni values, which according to Darwin are generally possible. 



