o98 E. C. ANDREWS THE HYPOTHESIS OF MOUNTAIN FORMATION 



Comhining these results, it is permissible, perhaps, to infer tentatively 

 that the earth shell, or. indeed, its whole mass, as Chamberlin suggests, 

 is separable into several large blocks of varying density. These blocks 

 have the same positions today, approximately, which they possessed in 

 Archeozoic time, but they have been growing increasingly stable and 

 fixed — that is to say. the Pacific area is becoming more established as a 

 definite basin than it was in Archeozoic time, and the continents also are 

 becoming more stable, with progress of time, by fusion and welding of 

 the old rings of land- wave growth. 



In the Archeozoic the crust appears not to have been so strong as it is 

 at present, and thus the reaction from the deeper portions of the zone of 

 flowage toward the surface, or region of negligible pressure, was more 

 readily attained at that period than now. Indeed, it is very difiicult to 

 conceive of a zone where definite rock flowage could occur unless there 

 should be an avenue of relative escape from the surroimding pressures. 

 If it be assumed that rock pressures at depth are enormous, but the pro- 

 viso be made also that they are equal in every direction and alike at all 

 points, this relative movement between great blocks appears impossible. 

 In nature, however, a decided oppoitunity for escape is supplied by the 

 negligible pressure at the earth's surface. On the assumption of a solid 

 earth, the only opportunity for the occurrence of a zone of rock flowage 

 at a moderate depth within the surface shell is due to the fact of negli- 

 gible pressure at the surface, where the rocks are composed of brittle 

 material of short grain and not of a coherent band or shell of elastic 

 material of long and strong bond, such as iron or copper. And this again 

 implies the non-existence of a zone of marked flowage in a solid earth at 

 a depth of hmidreds of miles below the surface, outside the hypothetical 

 contacts separating the great blocks themselves. The explanation is that 

 the strains at great depths could be distributed wholly below the surface, 

 and hence little or no opportunity for definite rock flowage would occur. 



This conception of an avenue of escape from pressure might be borne 

 in mind by students of d}Tiamics, whether of ordinary streams, of glaciers, 

 of rock flowage, or of igneous emanations; otherwise the main points at 

 issue may not only be misunderstood, but actually overlooked. 



The mighty mountain ranges of the earth, together with the igneous 

 rocks therein, are all results of this attempt at escape to the surface — 

 that is, to the zone of least pressure. 



C'hamljerlin's^^ conception of the initial blocks of variable density 



1* Chamberlin and Salisbury : Geology, 1906, vol. ii. pp. 126-130. On the other hand, 

 the snbaccordant depths of the oceans, in the main, indicate strongly that the earth's 

 surface is under one main control — that is, the zone of flowage. 



