£>IASTHOPHIC MOVEMENTS 259 



the globe. Under the nebular hypothesis a difficulty is here met which 

 has never been squarely faced, but, like so many other contradictions of 

 facts by the old hypothesis, has been neglected in the hope that some 

 time, somehow the explanation would be found. The amount of tan- 

 gential compression which the physicists allow to the crust of the cooling 

 globe is insufficient to account for the actual shortening which has oc- 

 curred in the making of the many mountain systems. Mallet estimated 

 that the earth had shrunk from the molten state 189 miles in diameter, 

 a circumferential reduction of less than 600 miles, and most of this must 

 have taken place during solidification and in the stages of high tempera- 

 ture. But nearly all the great existing mountain systems have been 

 formed since later Paleozoic time, since which time the secular contrac- 

 tion must have been very small. Claypole estimated the amount of 

 shortening in the Appalachians as 88 miles, and Heim estimates the 

 same effect in the Alps as 72 miles. 



Under the new hypothesis the earth shrinkage is due to original poros- 

 ity and gravitational compression and is in active operation today. The 

 amount of such spacial reduction has not been estimated, but it must be 

 several times any possible reduction due to cooling. The ocean repre- 

 sents elimination of interstitial space : so does the atmosphere, condensed 

 to solid or fluid, and also the unknown but enormous amount of gaseous 

 material which has escaped from the earth's control. To the contraction 

 represented by this vast amount of out-squeezed material must be added 

 the condensation of the interior mass as absolute molecular compression, 

 without extrusion of matter. This latter factor in the shrinkage of the 

 globe is the only one which the old hypothesis can recognize. 



The new view not only favors much greater amount of contraction in 

 the globe, but makes the rate of contraction more uniform throughout 

 geologic time. 



To whatever degree the lateral compressive strain in the earth's sur- 

 face layers may be due to insertion of wedges of igneous intrusives, the 

 new hypothesis is as favorable as the old. 



The up-and-down (epeirogenic) movements of continental areas are 

 difficult of present explanation under either hypothesis. They would 

 seem to have readier solution under the hypothesis which favors greater 

 movement in the mass of the globe. The new hypothesis would also 

 seem to be more favorable to changes produced by the invasion of the 

 cold superior strata by heat from beneath, and the .several possible re- 

 quirements of viscosity, or mobility, or potential fluidity of the interior 

 mass are equally well met by the new hypothesis. 



With the passing of the discredited nebular hypothesis substitutes 



XXXIV— Bull. Gkol. Soc. Am., Vol. 15, 1903 



