212 The American Geologist. ^i'"'- ^'^^^ 
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 sup- 
ply was correlated with a superior power of retention. 
The amount of oxygen in the early atmosphere is more un- 
certain 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 temperatures 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 pro- 
gressive 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 the earth's mass the required conditions would probably 
be reached. * * * 
Pressure itself is probably incompetent to melt rock sub- 
stances that shrink in solidifying, but the high temperatures gen- 
erated by pressure in the deep interior were constantly moving 
outward into horizons of lower pressures, where the melting- 
points were lower. As the computed temperature at the center 
