576 GEOLOGY 



of the past at least, and it is unknown whether there were elsewhere 

 ocean-belts well suited to the generation of large tides. The ocean- 

 tide, therefore, furnishes a very uncertain basis for estimating the 

 retardation of rotation/ The theroetical case rests largely on the 

 assumption of an effective body- tide. The earth doubtless has some 

 body-tide, but whether it is sufficiently great to be effective, and whether 

 its position, which depends on its promptness in yielding and in resili- 

 ence, is favorable to the retardation of rotation, are yet open questions. 

 The existence of an appreciable body-tide has not yet been proved by 

 observation. 



G. H. Darwin, assuming that the earth is viscous enough to give a 

 body-tide of appreciable value and of effective position, has deduced 

 a series of former rates of rotation of the earth and has computed the 

 corresponding distances of the moon. C. S. Slichter has shown that 

 the lessening of the area of the surface and the increase of the force of 

 gravity corresponding to these assigned changes of rotation are large, 

 and that if the changes were actually experienced they must have 

 involved much distortion of the crust. ^ These distortions would, how- 

 ever, be of a peculiar nature, and should thereby be detectible, if they 

 were realized ; for in passing from a more oblate to a less oblate spheroid, 

 the equatorial belt shrinks, and the polar tracts rise and become more 

 convex. Wrinkles should, therefore, mark the equatorial belts, and 

 tension the high latitudes. Slichter has computed that in a change 

 from a rotation period of 3 . 82 hours to the present one, the equatorial 

 belt must shorten 1131 miles and the meridional circles lengthen 495 

 miles. If we take Heim's estimate of the crust-shortening involved 

 in forming the Alps — 74 miles — as a standard, the 1131 miles of equa- 

 torial shortening would be sufficient for the formation of 15 mountain 

 ranges of Alpine magnitude. If, as some geologists urge, the estimate 

 of mountain folding is too great, the quotient would be still larger. 

 These ranges should rim across the equator and be limited to about 



^ The following conclusion by an eminent authority has come to our notice since 

 this was written : 



L' influence des marees oceanienes sur la duree du jour est done tout a fait minime 

 et n'est nullement comparable a I'effet des marees dues a la viscosite et a I'elasticite 

 de la partie solide du globe, effet sur lequel M. Darwin a insiste dans une series de 

 Memoires du plus haut interet. Par H. Poincare, Bulletin Astronomique, toma XX 

 (June, 1903), p. 223. 



' On the Secular Changes in the Elements of the Orbit of a Satellite revolving about 

 a Tidally-distorted Planet. Phil. Trans., Roy. Soc, Pt. II, 1880. 



'Jour. Geol., Vol. VI, 1898, p. 65. 



