Theories of the Earth's Origin — Upham. 213 
of the adult earth is about 20,000°' C, there would seem to be no 
lack of heat, in the later stages at least. The essence of the 
problem lies in its redistribution and in its selective action. 
The material of the interior was originally, by hypothesis, an 
intimate mixture of planetesimals of various kinds, with such 
gaseous material as they carried in or entrapped in the process 
of growth. * * * The outward flow of heat in such a mixture 
must bring some parts to fusibility much before the melting- 
points of other parts were reached. Local spots of fusion must 
thus arise. To this fusion the entrapped and occluded gases may 
be presumed to have contributed and to have joined themselves 
to the fused masses, and to have aided in giving them fluid- 
ity. * * * 
It is not necessary to the hypothesis to suppose that volcanic 
action was an essential preliminary to the acquisition of an at- 
mosphere, nor that it canie into function before the earth acquired 
an atmosphere, for the initial atmosphere may have been sup- 
plied from external sources. The apparent vigor and the wide 
prevalence of volcanic action on the moon, if its pitted surface 
means vulcanism, as well as the glassy material found in meteor- 
ites, whose origin is referred preferably to small atmosphereless 
bodies, favors the view that the internal gases were given forth 
abundantly before the earth grew to a mass sufiicient to hold 
.them. If this were true, an ample source of atmospheric supply 
was ready and waiting when the earth first acquired sufficient 
gravity to clothe itself with a gaseous envelope. 
When the increasing water-vapor of the growing atmosphere 
reached the point of saturation, it is of course assumed to have 
taken the liquid form and became a contribution to the hydro- 
sphere * * * 
If it be assumed that the earth's growing hydrosphere ap- 
peared at the surface when our planet had attained the mass of 
Mars, whose radius is about 2,100 miles, the subsequent growth 
would form a shell about 1,900 miles thick. It is not altogether 
ceftain that Mars bears water bodies on its surface; but the areas 
of greenish shade environed by a surface generally ruddy, the 
polar white caps ("snow caps") that come and go with the sea- 
sons, and the apparent occasional presence of clouds, not to appeal 
to the evidence of aqueous absorption lines in the spectrum re- 
ported by some good observers, but unconfirmed by others, lend 
some support to the opinion that water is present, though perhaps 
not in the form of definite water bodies. * * * 
Without attempting to fix the precise stage, it is not un- 
reasonable to assume that surface waters had begun their accu- 
mulation upon the earth's exterior while yet it lay 1,500 to 1,800 
miles below the present surface. The present difference between 
the radii of the oceanic basins and the radii of the continental 
platforms is scarcely 3 miles, on the average; so that if the con- 
tinental segments be assumed to be in approximate hydrostatic 
