Theories oi the Earth' s Origin — Upham. 211 
grew but little, have high eccentricities, as a rule. * * * 
To bring out the geological bearings of the planetesimal 
hypothesis, I have given considerable time to a study of the prob- 
able stages of growth of the early earth, of the time and mode 
of introduction of the atmosphere and hydrosphere, and of the 
initiation of the great topographic features, together with the 
leading modern processes. * * * 
Following the postulates of the previous sketch, a nebular 
knot is assumed to have been the nucleus of the growing earth. 
* * * Assuming that the nuclear mass was quite small, it is 
inferred that it was composed chiefly of matter of high molecular 
weight, since light molecules would be liable to escape because 
of their velocities. The nucleus is supposed to have been origin- 
ally an assemblage of planetesimals grouped together by their 
mutual gravity, and to have passed gradually into a solid mass 
in connection with the capture of outside planetesimals. * * * 
As the solid nucleus thus formed may not have been massive 
enough to • control a gaseous envelope in its earlier stages, a 
possible atmosphereless stage is to be recognized. Just how 
massive a planetary body must be to hold permanently an appre- 
ciable atmosphere is not accurately computable at present, be- 
cause of the uncertain value of some of the factors involved. A 
fairly safe conclusion may perhaps be drawn from known celestial 
bodies. The moon * * * has no detectable atmosphere, nor 
has any smaller body, whether satellite or asteroid, so far as 
known. ' Mars * * * has an appreciable, but apparently quite 
limited, atmosphere. The limit between atmosphereless and 
atmosphere-bearing bodies probably lies between the twj— i. e., 
roundly between one-eightieth and one-tenth of the earth's 
mass. * * * 
When the growing earth reached a mass sufficient to control 
the flying molecules of atmospheric material, there were two 
sources from which these could be supplied for the accumulation 
of an atmosphere, an external and an internal one. * * * 
In the later stages of organization, and thence down to th$ 
present time, the molecules discharged from all the bodies of the 
solar system were possible sources of atmospheric accretion. Of 
these the most important were probably volcanic and similar dis- 
charges from the small bodies that could not hold gases perma- 
nently and discharges from the sun by virtue of the enormous 
explosive and radiant energies that are there resident. 
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 mate- 
rial. * * * 
The gases chiefly occluded in meteorites and the crystalline 
rocks are hydrogen, carbon dioxide, and carbon monoxide in 
