EARLY THEORIES CONTRACTIOX 343 



A modification of this theory of higher speed of rotation calls for the 

 slowing down of the speed with a consequent approach to more nearly 

 spherical form. Direct results of this process would be a vast series of 

 folded ranges trending north and south across the equator and also a 

 tremendous stretching of the crust in the vicinity of the poles. Since 

 the crust of the earth shows no such plan, the theory fails to meet the 

 test of facts in geography. Furthermore, it fails to meet the facts in 

 point of time, for such a slowing down of the earth's revolution must 

 have been gradual and its effects to a considerable extent rhythmical. 

 No such rhythm is observable, however, for the greatest deformations of 

 the earth, those of Precambrian, Paleozoic, and Tertiary time, are sepa- 

 rated by enormously different intervals. 



Another cause assigned is that of variation of the attitude of the 

 earth's axis, with attendant changes in the crust of the spheroid. While 

 variations appear to exist, they are not known to be large, and there is 

 no demonstration, or perhaps even indication, that they affect mountain 

 structure. In short, none of the theories of mountain-building which 

 depend for their causes on factors of an astronomical nature satisfy the 

 facts of geology and the calculations of physicists and astronomers. 



Among the various theories which depend in part on facts of observa- 

 tion, the oldest is that of radial contraction of the earth due to cooling. 

 Others are the theory of a tetrahedral form as a result of contraction ; 

 the theory of isostatic adjustment between crustal masses of unequal 

 density ; the theory of suboceanic spread, in which the suboceanic por- 

 tions of the crust are thought to be denser than those beneath the conti- 

 nents, which relation causes a flow of the suboceanic mass toward the 

 continents so as to equalize the masses. All of these theories depend on 

 gravity as the moving force. 



CONTRACTION 



The hypothesis that mountain-folding was caused by shrinkage of the 

 earth on account of loss of heat has been discussed and supported by 

 many eminent geologists. It is based on the fact that the earth radiates 

 heat faster than it receives it from the sun, and on the supposition that 

 there is no other appreciable source of heat. It is in addition based on 

 the theory that the earth has been derived from a nebulous mass through 

 various stages of condensation. The actual attractions of the nebulous 

 particles for each other were finally combined in the solid earth into an 

 effective force called gravity. Since the earth actually loses heat by 

 radiation, it must slowly condense and contract. This change of volume 

 is most rapid at high temperatures, so that the cool outside or cru^t of 



XXIII — Bull. Gkol. Soc. A.m.. Vol. 84, 1022 



