■J 



416 A. Blytt on the probable Cause of 



In tlie Philosophical Transactions for 1879, Parts I. & II., 

 Prof. G. Darwin has published a memoir the results of which 

 are briefly as follows. 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 ecliptic has diminished, 

 and that a great part of the internal heat is developed by the 

 internal friction. 46,800,000 years ago, according to his 

 calculation, the sidereal day was 15 h. 30 m., 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 h. 45 m. 

 long, the moon's distance only 9 terrestrial radii, and the 

 month only 1*58 day {-^ 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.* He 

 concludes that the compression has constantly diminished : — 

 " the polar regions 7nust have been ever rising, and the equato- 

 rial 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, 

 ;ind accordingly the rate of oceanic denudation much acce- 

 lerated. The more rapid alternation of day and night 

 [57,000,000 years ago, according to Darwin, the year had 

 1300 days] would probably lead to more sudden and violent 

 storms ; and the increased rotation of the earth would aug- 

 ment the violence of the trade-winds, which, in their turn, 

 would affect oceanic currents|. 



Tresca {Comptes Reiidus, 1864, p. 754; 1867, p. 802, &c.) 



diminishing centrifugal force lias produced foldings in a north and south 

 direction. J. E. Todd, in a paper entitled " Geological Effects of a vary- 

 ing Rotation of the Earth " (Amer. Naturalist, vol. xvii. 1883, pp. 15 et 

 seqq.), first enumerates the various forces which may act in accelerating 

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

 decreases and increases abruptly, that it acts tirst upon the sea and after- 

 wards upon the sohd crust, and that for this reason the sea rises and 

 sinks abruptly in relation to the land. 



* This heat, produced by the internal friction, must contribute con- 

 siderably 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. 



t In a subsequent article, however, Parwin supposes that the coast- 

 lines will shift in consequence of the lengthening of the sidereal day 

 (' Nature,' Sept. 2, 1886, p. 422). 



J The numerical values given above make no claim to represent the 

 actual values ; they are merely the maximum values, which, according 

 to Darwin, are generally possible. 



