118 GEOLOGY. 



of the enormous granitic batholiths of the Archean were in themselves 

 a source of deformation. Great changes in the distribution of tem- 

 perature were involved in the outward flow of the molten rock. This 

 carried heat outward from the deeper zones, and expanded the rock 

 in the zones through which the rising lava passed. There was, there- 

 fore, loss of both material and heat in the lower zones, and gain of 

 material and heat near the surface, with the obvious effects. The 

 principles set forth in Vol. I, pp. 564-568, relative to the general transfer 

 of heat by conduction and its double effects were also in play. In 

 harmony with this intensive combination of agencies, the Archean 

 rocks, as we shall see in the next chapter, are extraordinarily folded 

 and crumpled, and at the same time phenomenally traversed by intrusive 

 sheets, tongues, and embossments of plutonic rock. Diastrophism 

 probably had its climax with the climax of vulcanism, and both came, 

 by hypothesis, about the time of the opening chapter of the revealed 

 history of the earth. 



It is at this stage, and with the great agencies thus related to 

 One another, that we reach the lowest observable formations of the 

 earth's body, in following the accretion hypothesis. It is for each 

 student to judge for himself whether the postulated antecedents lead 

 felicitously or otherwise into the actual state of things which the 

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

 stration of its truthfulness is not yet possible, lies in its working qualities. 



VIII. The Great Stage of Atmospheric and Hydrospheric 



Dominance. 



To complete the survey of stages, it is necessary to glance forward 

 through the remaining ages and note the great fact that, the growth 

 of the earth having ceased, and volcanic action having passed its climax, 

 the surface of the land was no longer subject to continual burial, but 

 was exposed, age after age, to the action of the atmosphere and the 

 attendant work of streams and ground- waters. The material removed 

 by these agents was deposited in the basins. Throughout this stage, 

 therefore, the dominant phase of geologic activity became the hydro- 

 atmospheric or gradational, as nearly as it can be expressed or implied 

 in a single word. Vulcanism and diastrophism continued to be very 

 important accessories, but not the controlling factors. This stage 



