196 WARREN UPHAM, M.A., F.G.S.A., ON THE NEBULAR AND 



"As the planetesimals were gathered into the growing earth- 

 nucleus they carried their occluded gases in with them, except as 

 the superficial portion might be set free by the heat of impact. 

 There was thus built into the growing earth atmospheric material. 



" The gases chiefly occluded in meteorites and the crystalline 

 rocks are hydrogen, carbon dioxide, and carbon monoxide in 

 leading amounts, and marsh-gas and nitrogen in small quantities. 

 It is assumed that the gases of the aggregated planetesimals, and 

 hence those of the interior of the early earth, were of the same 

 order of abundance. . 



" In determining the actual proportions of the constituents of the 

 early atmosphere, the abundance of the supply was probably less 

 decisive than the power of the earth to hold the individual gases. 

 As gravity gradually increased by the growth of the earth from an 

 incompetent minimum, its power to control the heaviest molecules 

 with the lowest velocities was acquired before its ability to hold 

 the lighter ones of higher velocities. . 



" Carbon dioxide would be held some appreciable time before 

 oxygen, and still longer before nitrogen, and all these a notable 

 time before the vapor of water. The inference is that the initial 

 atmosphere was very rich in carbon dioxide, for an abundant 

 supply was correlated with a superior power of retention. 



" The amount of oxygen in the early atmosphere is more 

 uncertain, from doubt as to a competent source of supply . . 

 For the primitive atmosphere there is theoretical need for only 

 enough oxygen to support the primitive plant life until it could 

 supply itself, after which it would produce a surplus . . . 



" After the earth acquired the power of holding water- vapor, 

 the supply being abundant, accession doubtless went on for a time 

 as fast as the capacity to hold increased. 



" The problem of vulcanism assumes a quite new aspect under 

 the planetesimal hypothesis, if very slow accretion without very 

 high temperature be assumed. It has been taken for granted in the 

 preceding statement that there was volcanic action. It is necessary, 

 therefore, to consider how volcanic action may have arisen, and this 

 involves the more radical question how the high internal tempera- 

 tures of the earth may have arisen if the earth did not inherit its 

 heat from a molten condition arising from a gaseous origin . . . 



" The chief source of internal heat is assigned to the progressive 

 condensation of the growing body as material was added to its 

 surface. The amount of this condensational heat for the full- 

 grown earth, computed on the best data now available, seems to be 

 ample to meet all the requirements of the known geologic ages 



. . . That heat arising from condensation solely would reach 

 the melting temperature of rock in a body one-twentieth of the 

 earth's mass, seems more or less doubtful, but in a body one-tenth of 



