344 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



the earth is relatively more fixed in condition than the interior and is 

 wrinkled on the more rapidh' condensing interior. 



If the continuing loss of heat of the earth without replenishment is 

 granted, there must be condensation and wrinkling of the crust. Grave 

 doubt has been thrown on the fundamental assumption as to loss of heat 

 by the planetesimal hypothesis of Chamberlin, which calls for a stead}^, 

 though small, accession of meteoric matter that gives on impact some 

 accession of heat. A further serious blow has been dealt to this assump- 

 tion by the recent discoveries in radioactivity. Long before the theory 

 had to meet these modern doubts, it encountered several major difficul- 

 ties which were necessar}' consequences of the theory. 



Important among these obstacles is that of periodicity, which has 

 been a stumbling block to many other theories of mountain-building. 

 If mountains were raised by contraction due solely or mainly to loss of 

 heat, then the strains began and increased regularly. Since their relief 

 by deformation was determined by the strength of the rocks, and since 

 this strength can not be considered to have varied from age to age, it 

 follows that the stress should have increased rapidly to a certain point 

 controlled by the rocks themselves and then been satisfied by deforma- 

 tion. Then the process would have begun over again, and a notable 

 periodicity in mountain-building would have certainly followed. There 

 are those who advocate periodicity and rhythm in movements of the earth 

 which were sufficient to produce important changes in the distribution 

 of the sea and its animals, now recognizable in terms of era and system. 

 There is no demonstration, however, that the various eras are of equal 

 length or separated by equal movements. Far less is it the fact that the 

 great mountain-building movements like that of the Appalachians are 

 separated by equal j^eriods or were of equal intensity. The tremendous 

 difference in intervals and energy between the Devonian and Permian 

 mo\'ements in the Appalachians is proof that they were not equal. In 

 this general respect the theory fails. 



A deficiency of this theory, which is perhaps the principal one, is of a 

 quantitative nature. In order that the folded mountain ranges may 

 have been produced as the result of contraction, the radial movements 

 must have l)een translated into horizontal movements of the crust. Many 

 mathematical calculations have been made as to the time needed for con- 

 traction through loss of heat to produce the observed horizoiital move- 

 ments in a given mountain range. These calculations have been based 

 on such estimates of horizontal shortening as 40 miles for tlie Appa- 

 lachians and 75 miles for the Alps. Even on the basis of such shorten- 

 ing it has been recognized by geologists that the strain put on the theory 



