C. Lloyd Morgan — Geological Time. 157 



the earth's surface, and will cause '•' the earth constantly to turn the 

 same portion of its surface towards the moon, and therefore to rotate 

 about its axis in the same period as that in which the moon revolves 

 about it. This most remarkable ultimate effect we see already pro- 

 duced in the moon — it is precisely the same thing — we see the 

 moon turning almost exactly the same portion of its surface to the 

 earth at all times." 



" It being thus established that the rate of rotation of the earth 

 is constantly becoming slower, the question comes : How long ago 

 must it have solidified in order that it might have the particular 

 amount of polar flattening which it shows at present ? " To which 

 Professor Tait gives the somewhat indefinite answer: "That because 

 the earth is so little flattened it must have been rotating at very 

 nearly the same rate as it is now rotating when it became solid. 

 Therefore, as its rate of rotation is undoubtedly becoming slower and 

 slower, it cannot have been many millions of years back when it 

 became solid, else it would have solidified into something very much 

 flatter than we find it. That argument, taken along with the first 

 one, probably reduces the possible period which can be allowed to 

 geologists to something less than ten millions of years." 



Now, even if we grant, as is quite probable, that, when it first 

 solidified, the earth was a more oblate spheroid than at present, 

 it is by no means probable that she would have continued to possess 

 the self-same figure until now ! Sir William Thomson himself and 

 Mr. George Darwin, in their papers on the possible changes of 

 the earth's axis of rotation, express a belief that the earth would 

 readjust itself to its form of equilibrium, if by some chance it should 

 at any time fail to conform to that figure. This might be effected 

 either " impulsively by means of earthquakes," or gradually, in 

 which latter case there might have been a special tendency for the 

 melted products of volcanic action to be extruded in the neigh- 

 bourhood of the polar regions ; and it is a noteworthy fact that 

 polar lands are remarkably volcanic in their character. No geolo- 

 gist who has studied the contortions of strata in any mountain 

 region could believe that, since its first solidification, the earth's 

 figure had been absolutely unaltered, while many would contend, 

 and with much weight, that whatever its original form, during the 

 mechanical deposition of strata its present figure must have been 

 produced. " We cannot infer from the present shape of our globe," 

 writes Mr. Croll, " what was its form, or the rate at which it was 

 rotating at the time when its crust became solidified. Although it 

 had been as oblate as the planet Jupiter, denudation must in time 

 have given it its present form." Polar ice-caps and the varying 

 distribution of land and ocean introduce uncertainties into the 

 data for calculation, which must prevent our placing any implicit 

 faith in the results obtained. 



The third point taken into consideration by Sir William Thomson, 

 in his estimate of the length of geological time, is the probable 

 duration of the sun's heat. There are several theories of the 

 origin and source of solar heat. Among others, as indeed we might 



