434 GEOLOGY 



the continental protuberances began almost as soon as the hydro- 

 sphere began to gather, which was long before the earth had reached 

 its present size, and has continued to the present time. 



(5) The initial life stage. Suitable conditions for life did not 

 exist until after some notable development of the atmosphere and 

 the hydrosphere, but as these were gathered about the earth at an 

 early stage, it is possible that some forms of life began long before 

 the earth was full-grown. Under the planetesimal hypothesis, 

 therefore, the time during which life may have existed on the earth 

 is very much longer than the time assumed under the older 

 hypotheses. 



(6) The climax of volcanic action. While volcanic action may 

 have begun soon after the beginning of the earth's growth, it prob- 

 ably had to await (1) sufficient growth to give the requisite heat 

 by compression, and (2) sufficient time for the heat so developed 

 to creep out to zones of less pressure, where it would suffice 'to 

 liquefy the more fusible (soluble) parts of the rock. Vulcanism 

 was probably hastened by radioactivity. Once begun, it is be- 

 lieved to have gradually increased in importance, and only reached 

 its climax some time after the more rapid growth of the earth had 

 ceased. 



For obvious reasons, the climax of vulcanism was attended by 

 deformations of exceptional intensity. The transfer of so much 

 material from below to the surface required readjustment within, 

 and the intrusion of the enormous granitic batholiths, such as are 

 found in the early formations, was in itself a cause of deformation. 

 Diastrophism probably had its climax with the climax of vulcanism, 

 and both came, by hypothesis, about the time of the opening 

 chapter of the well-recorded history of the earth. 



The formations of the period when volcanic action was at its 

 height, including some contemporaneous sedimentary deposits, are 

 regarded as constituting the oldest accessible rocks of the earth 

 (the Archean Complex), though probably only the later part of 

 the great volcanic series is represented by the known Archean. It 

 is for each student to judge whether the assigned antecedents lead 

 felicitously or otherwise into the actual state of things which the 

 oldest known rocks reveal. The value of a hypothesis, when its 



