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20 COSMICAL ASPECTS OF GEOLOGY. — [Boox I. 
That the formerly larger amount of solar heat received by the 
surface of our planet must have produced warmer climates and more 


rapid evaporation with greater rainfall and the important chain of 
geological changes which such an increase would introduce, appears: 
in every way probable, though the geologist has not yet been able 
to observe any indisputable indication of such a former intensity of 
superficial changes. 
Mr. George H. Darwin, in recently investigating the bodily tides of | 
viscous spheroids, has brought forward some remarkable results 
bearing on the question of the possibility that geological operations, 
both internal and superficial, may have been once greatly more gigantic 
and rapid than they are now.’ He assumes the earth to be a homo- 
geneous spheroid and to have possessed a certain small viscosity,” and 
he calculates the internal tidal friction in such a mass exposed to the 
attraction of moon and sun, and the consequences which these bodily 
tides have produced. He finds that the length of our day and month 
have greatly increased, that the moon’s distance has likewise 
augmented, that the obliquity of the ecliptic has diminished, that a 
large amount of hypogene heat has been generated by the internal 
tidal friction, and that these changes may all have transpired within — 
comparatively so short a period (57,000,000 years) as to place them 
quite probably within the limits of ordinary geological history. 
According to bis estimate, 46,300,000 years ago the length of the 
sidereal day was fifteen and a half hours, the moon’s distance in mean 
_ radii of the earth was 46°8 as compared with 60-4 at the present time. 
But 56,810,000 years back the length of the day was only 6% hours, 
or less than a quarter of its present value, the moon’s distance was only 
nine earth’s radii, while the lunar month lasted not more than about 
a day and a half (1°58), or -1. of its present duration. He arrives at 
the deduction that the energy lost by internal tidal friction in the 
earth’s mass is converted into heat at such a rate that the amount 
lost during 57,000,000 years, if it were all applied at once, and if the 
earth had the specific heat of iron, would raise the temperature of the 
whole planet's mass 1,700° Fahrenheit, but that the distribution of 
this heat generation has been such as not to interfere with the normal 
augmentation of temperature downward due to secular cooling, and 
the conclusion drawn therefrom by Sir William Thomson. Mr. 
Darwin further concludes from his hypothesis that the ellipticity of 
the earth’s figure having been continually diminishing, “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. The tides must have been very 
1 Phil. Trans., 1879, Parts i. and ii. 
* The degree of viscosity assumed is such that “ thirteen and a half tongs to the 
square inch acting for twenty-four hours on a slab an inch thick displaces the upper 
surface relatively to the lower through one-tenth of an inch. It is obvious,” says Mr. - 
Darwin, “that such a substance as this would be called a solid in ordinary parlance, 
and in the tidal problem this must be regarded as a very small viscosity.” Op. cit. 
p-. 551. 
