430 GEOLOGY 



The hypothesis of an enormous original atmosphere, suffering 

 gradual depletion, finds, therefore, scant support in a critical study 

 of either the biological or the physical history of the earth. 



II. POSSIBLE MODIFICATION OF THE PRECEDING VIEW 



The most troublesome phases of the preceding scheme arise 

 from the assumption that the lighter gases were excluded from the 

 hot molten globe, and so formed a vast atmosphere. If it is possible 

 to amend the hypothesis, it must be done, apparently, by assum- 

 ing that the earth held large quantities of gaseous constituents 

 throughout the molten stage, and discharged them later. Since 

 lavas now bring to the surface great volumes of absorbed gases, 

 it is not, on its face, inconsistent to suppose that a globe of molten 

 rock might contain large quantities of the atmospheric gases. It 

 is perhaps admissible to assume further that some notable part of 

 the atmospheric material remained in the rock after its solidifica- 

 tion, since igneous rocks now contain notable quantities of gas. By 

 making these assumptions, the primitive atmosphere may be held 

 to have been less extensive than in the preceding view, and gaseous 

 material may be supposed to have been held in reserve in the earth- 

 body to actuate future vulcanism and to feed the atmosphere and 

 hydrosphere. 



This modified view makes it possible to suppose that the forma- 

 tion of the crust may have been followed by a period of exceptional 

 volcanic activity, during which the primitive shell was buried so 

 deeply under volcanic matter that it has not since been exposed by 

 erosion. It is consistent then to suppose that the oldest rocks 

 accessible are these volcanic products, mingled with such sedi- 

 mentary material as was formed contemporaneously with them. 

 In this way the hypothesis may be made to fit the earliest rocks 

 now known, fairly well. It is not clear, however, that the physical 

 assumption on which it is based is sound, for experimental evidence 

 seems to indicate that highly heated rock material discharges its 

 gases, rather than retains them. 



The modified hypothesis is only partially successful in meeting 

 the atmospheric difficulties, for there must have been added to the 



