HYPOTHETICAL STAGES LEADING CP TO THE KNOWN ERAS. 99 



IV. The Ixitiatiox of Vulcanism. 1 



\Vhile some extrusion of the gases condensed within the rock-body 

 of the earth may take place quietly and continuously, as the result 

 of progressive contraction and of the rearrangement of material, and 

 by means of diffusion and like processes, there is at present little or 

 no observational ground for affirming that this is a process of much 

 moment. It is, however, held to be such by some eminent geolo- 

 gists. 2 The recent discoveries of radio-active emanations from the 

 earth and from ground-waters are at least interesting in this connec- 

 tion, though it is not now known what permanent contribution, if any, 

 these emanations make to the atmosphere. 



Vast quantities of gas are, however, shot forth by volcanoes, or 

 more gently extruded from lavas. This is the chief demonstrable — 

 as it is the only demonstrative — mode by which internal gases are 

 given forth. It is by no means the universal opinion of geologists 

 that these are really interior gases, coming for the first time to the 

 surface (Vol. I. pp. 621-622, 633-636); but the accretion hypothesis 

 assumes that this is the case for a large part of these gases, without 

 excluding the participation of surface-water and atmosphere in pro- 

 ducing some of the volcanic gases. 



The -sources of the requisite heat. 



If it be granted that the growing body contained abundant atmos- 

 pheric materials, the critical question arises whether an adequate de- 

 gree of heat would be developed within the earth to actuate vulcan- 

 ism and force the gases to the surface, when slow accretion is the 

 mode of growth assigned. The sources of heat and the degree of 

 their availability must, therefore, be considered. 



1 Xot improbably vulcanism was initiated before the gravity of the earth enabled 

 it to hold an external atmosphere, as may be inferred from the phenomena of the 

 moon, but it is more convenient to treat vulcanism after the sources of atmospheric 

 material have been considered. In the main, the growth of the atmosphere and of 

 the hydrosphere ran hand in hand with the development of vulcanism, under this 

 hypothesis, and the precise order of initiation of the atmosphere and of vulcanism 

 is immaterial. If the agencies of vulcanism did not bring it into action until after 

 the earth acquired the ability to hold an atmosphere, the latter would begin to be 

 collected from without. 



2 Suess, Geogr. Jour., Vol. XX, 1902, p. 520. 



