386 THE PLAN OF THE EARTH AND ITS CAUSES. 



This is important because, as Lord Kelvin lias shown, the amount of 

 contraction allowable during the later stages of the earth's history is 

 very limited. But geologists have the authority of Professor Darwin 

 for accepting a certain amount of contraction. "A cooling celestial orb 

 must contract by a perceptible fraction of its radius after it has consoli- 

 dated," he tells us, and his considerations " only negative the hypothesis 

 of any large contraction of the earth since the moon has existed." ] And 

 unlike the contraction theory of the origin of mountain chains, the 

 theory of the tetrahedral deformation of the lithosphere requires only 

 a small amount of radial contraction. 



Finally, it may be urged that even such deformation as the tetrahe. 

 dral theory requires is impossible, since physicists have taught us that 

 the earth is rigid. To this objection it is only necessary to reply that 

 Lord Kelvin's rigidity arguments apply to the earth as a whole, and 

 not to its crust; they deny the fluidity of the interior of the earth, and 

 do not prohibit any local deformations of the exterior crust. The once 

 prevalent astronomical belief in the absolute invariability of the earth's 

 shape and in the absolute fixity of its axis of rotation (expressed, 

 e. g., by Sir J. Herschel in 1862) no longer hinders progress. In fact, 

 astronomers tell us that, instead of the absolute fixity of the pole, it 

 now shifts its position to an appreciable extent under the influence of 

 the movements of the atmosphere, the unequal melting of the polar ice, 

 and by heavy fails of snow on the Siberian highlands. These move- 

 ments of the j)ole are important, because they are taken to prove a 

 certain elasticity in the earth. The movements demonstrated by actual 

 observations are so far minute; but they at least allow geologists to 

 say that, as such slight causes as those mentioned produce appreciable 

 effects, more powerful causes acting for longer periods would work 

 greater changes. 



SUMMARY. 



The object of the paper is to show that the old belief in a definite 

 plan of the earth is justified, since the distribution of land and water on 

 the globe has been determined by the tetrahedral arrangement of the 

 elevations and depressions in the surface of the lithosphere. 



This tetrahedral plan is shown by the existence of (1) a northern 

 land belt, surrounding anortheru ocean, and giving off three meridional 

 land lines, which taper southward ; (2) the southern ocean belt sur- 

 rounding a south polar continent, and the three meridional oceans; (3) 

 by the antipodal position of land and water; (4) by the course of the 

 main watersheds and mountain chains. 



It is held that this arrangement was not established in the earth's 

 infancy, and therefore has to be attributed to some agency which has 

 acted throughout geological history. 



There are reasons for believing that a contracting sphere with a hard 



1 Phil. Trans., vol. 170, pp. 522, 523. 



