DIASTROPHISM AND THE FORMATIVE PROCESSES 695 



long flights in the ultra-atmospheric field of the sun, and if the 

 porous surface of the moon received and held by adsorption, chemi- 

 cal combination, or otherwise, molecular planetesimals of the gas- 

 forming order, such as would inevitably plunge into it from the 

 interplanetary field, there would be entrapped in the body of the 

 moon, as it grew, a supply of disseminated gas-producing material 

 sufficient to actuate great explosions whenever concentrated later 

 by conditions favorable for such action. As the moon grew, its 

 self-compression and the strains developed within it by neighboring 

 bodies should have forced this potentially gaseous material toward 

 the surface and developed the conditions of eruption. The moon 

 should also have inherited its quota of radioactive substances and 

 these should have played their part in the lunar vulcanicity. The 

 fragmental constitution of the outer part of the moon, postulated 

 as an inevitable feature of an accretional origin, should have 

 rendered it specially susceptible to explosive effects. More or 

 less local lava-production, as well as the quiet type of vulcanism, 

 are entirely consistent with this view of the exceptionally gaseous 

 eruptions, and they are postulated, but the evidence of the moon's 

 surface seems to give this more quiet action a place quite subordi- 

 nate to the gaseo-explosive phase. 



THE SIGNIFICANCE OF TERRESTRIAL VULCANISM 



The inferences that seem so imperative in the case of the moon 

 apply also to the gaseous phases of vulcanism on the earth. The 

 argument is a little less imperative because the earth is able to 

 hold an atmosphere and would probably do so to some less extent 

 in a molten state, and so, if it were once in that state, volcanic 

 gases could have been retained in its liquid mass sufficient to 

 balance the partial pressures of the like gases in such lessened 

 atmosphere as the earth then held. The amount of gases so held 

 in equilibrium could not have been large, and such as existed 

 would not have served as explosive agencies because of the very 

 fact that they were held in balance by opposing pressure. They 

 were there merely because there was an outside pressure holding 

 them there and that outside pressure was never removed under nor- 

 mal conditions. It seems merely declaiming the obvious to say that 



