TIME OF MOST EFFECTIVE DIASTROPHIC ACTION 207 



involves the distribution of diastrophic effects which depends on other 

 considerations than those under study here. The observational evidence 

 respecting the Archean Era, whose terranes are closest of kin to those of 

 the formative stages, seems to support the view that the surficial diastro- 

 phism was then greater than during any period of equal length in later 

 time. 



Bearings of the planetesimal View on Vulcanism 



Under the planetesimal view, vulcanism, as recorded in the geologic 

 ages, arose chiefly from th6 partial selective liquefaction of eutectic ma- 

 terials as they happened to be in contact here and there in the body of 

 the earth. The squeezing of this mobilized material to or toward the 

 surface by the differential pressures engendered within the earth-body, 

 including the graded stresses, more intense below than above, imposed by 

 outside bodies, was the main extrusive factor of the case. Vulcanism is 

 thus in itself little short of a phase of diastrophism. The heterogeneity 

 of the earth-substance, the inequalities of its distribution, and the preva- 

 lence of greater stresses below than above combined to promote both 

 processes and make them helpful companions in action. The distortions 

 of the deformative process are the natural counterparts of the partial 

 mobilizations and displacements of the volcanic, process. 



In a gaseo-molten earth, all of whose material passed gradually from 

 very high to much lower temperatures, while the mass was stirred by con- 

 vection at all stages, there should have been developed and brought to 

 the surface all the gaseous material in the earth-substance which high 

 heat could set free. When these hot gases entered the hot atmosphere 

 their molecular velocities should have been at a maximum, and the rate 

 and the extent of their escape from the atmosphere should have exceeded 

 that of any later time. When the long boiling process ceased and the 

 encrusting of the liquid sphere began, there should have been left in the 

 mass a minimum of gaseous material. Only that small measure which 

 might be held there by the partial pressure of like gases in the atmosphere 

 would be assignable under these conditions. The earth-body should then 

 have been scantily sujyijlied with explosive volcanic gases. It would seem, 

 therefore, that an earth-body formed in this way should be unsuited to 

 give rise to such a degree of gaseous vulcanism as is actually manifested. 

 Just as a once molten earth must have exhausted its main resources of 

 diastrophism before a record of diastrophism could be made, so it must 

 have exhausted in its liot youth essentially all its resources of explosive 

 vulcanism before a solid environment was provided to give effect to its 

 explosive possibilities. 



