200 The Field of Geology— McGee. 



and here the most profound of the remaining mysteries of 

 geology is found. 



It is only within a decade that diastatic movements of the 

 consequent class have been separated from the primary class; even 

 yet there are geologist^! who do not recognize the distinction; and 

 so most of tlie hypotheses thus far framed to explain the deforma- 

 tion of the terrestrial shell rest on the implicit or explicit assump- 

 tion that all diastatic movements belong to the class here called 

 antecedent. 



The primitive hypothesis ascribed the corrugations of the 

 terrestrial crust to more rapid contraction of the interior of the 

 earth than the exterior shell, accompanying secular cooling. The 

 common conception as to the mechanism of this process was fa- 

 miliarly illustrated by likening the corrugated globe to a withered 

 apple — the inequalities of the terrestrial surface corresponding to 

 the wrinkles on the apple's skin; and to the surprise of a majority 

 of geologists this hypothesis has been prominently advocated 

 within a year or two. It appears, however, quite untenable: 

 Fisher and others have shown that the postulated cause is far 

 from commensurate with the observed effect — that even upon the 

 most liberal estimates of radial contraction due to secular cooling, 

 the concomitant tangential contraction would not produce a tithe 

 of the observed corrugation of the terrestrial crust; Button main- 

 tains that equitable contraction of a spheroidal segment would not 

 produce corrugations such as those characterizing the earth's skin; 

 Taylor, Alexander Winchell and others have pointed out that any 

 corrugations resulting from secular contraction of the terrestrial 

 crust in combination with stresses resulting from precession, nu- 

 tation, retardation of axial rotation, etc., would tend to assume 

 certain definite directions, and that these directions do not coincide 

 with those of the mountain ranges actually existing nearly enough 

 to give countenance to the hypothesis; Reade and others have re- 

 cently discovered that tangential contraction due to secular cool- 

 ing must have been confined to a limited shell (even thinner than 

 the strata actually known to be corrugated); and it might be 

 shown that the concentration of montanic corrugati n along cer- 

 tain lines, leaving vast intervening areas quite undisturbed, does 

 not agree with the hypothesis and could not occur in accordance 

 with it under any conditions of rigidity and internal friction of 



