3 i2 EARLY STAGES OF EARTH'S HISTORY 



tended toward higher specific gravities, and hence toward still 

 further depression when deforming stresses were brought to bear 

 on them, while the elevated areas tended to grow relatively lighter, 

 and to suffer relative elevation, under the stress of deformative 

 movements. Thus the differentiation of the oceanic basins from 

 the continental masses began as soon as the hydrosphere began, that 

 is, long before the earth reached its present size, and has continued 

 to the present time. 



5. Initial life stage. Suitable conditions for life seem to have 

 existed after an atmosphere and hydrosphere had developed to the 

 proper extent, but it seems possible that 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. Climax of volcanic action. While volcanic action may have 

 begun early, it probably had to await (i) sufficient growth to give 

 the requisite internal 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 radio-activity. 

 Once begun, it is believed to have increased in importance, reaching 

 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 con- 

 temporaneous sedimentary deposits, are regarded as constituting 

 the oldest accessible rocks of the earth (the Archean}, though prob- 

 ably 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 to the condi- 

 tions which the oldest known rocks reveal. The value of a hypothe- 

 sis, when its truth cannot be demonstrated, lies mainly in its work- 

 ing qualities. 



