for the Study of Megadiastrophism. 409 



pliism should grow progressively less after cessation of 

 growth. But this conclusion is difficult to reconcile with 

 Genozoic deformation which far transcends all other post- 

 Proterozoic orogeny. 



On the other hand, with a slowly thickening crust rest- 

 ing on a yield zone (asthenosphere), crustal yielding, both 

 in time and degree, is a factor of crustal strength. 

 Periodicity of deformations in crescendoes to a high point 

 indicate increasing crustal strength and so an increasing 

 capacity for accumulating stresses before the breaking 

 point is reached. Under this view the pre-Paleozoic 

 deformations represent the rather easy yielding of a com- 

 paratively thin crust and its widespread magmatic engulf - 

 ment. From the viewpoint of an earth whose diastrophic 

 energies arise in the deep interior it is rather difficult to 

 explain periodic deformation, for the forces must be 

 continuously active and, since the surficial portions of 

 such an earth would be weaker than any underlying por- 

 tions, deformation could hardly result from accumulative 

 stresses. 



Even if the postulated method of very slow growth by 

 planetesimal accretion with its consequent state of abso- 

 lute solidity for the earth be accepted, the existence of 

 any great reserve of diastrophic energy may be seriously 

 questioned. If this growth took three or four billion 

 years it would seem that this would give ample time for 

 a very complete inner reorganization pari passu, leaving 

 an earth practically immune to deformational stress. 



The Earth's Tolcanism and tine Evidence of the Igneous Rocks. 



Volcanism, of course, using the term in its broadest 

 sense, finds little place in the groundwork for studies in 

 megadiastrophism. It is considered as merely an ' ' acces- 

 sory" to a " profoundly metamorphic earth." A once 

 molten earth would, according to Chamberlin, not only 

 waste its dynamic energy and become diastrophically 

 sterile but "there should have been developed and 

 brought to the surface all? 1 the gaseous material in the 

 earth substance which high heat could set free," and 

 "an earth-body formed in this way should be unsuited 



3T The italics are mine — W. F. J. 



Am. Jour. Sci.— Fifth Series, Vol. Ill, No. 18.— June, 1922. 

 29 



