96 The American Geologist. February, 1904. 
The old hypothesis assumes an originally hot globe, with 
shrinking on acount of cooling; the new regards the globe as 
originally and always cold at the surface, and the interior heat 
as the product of gravitational condensation. The old view 
requires continuous cooling of the globe, while the new allows 
the conception of increasing internal heat. The old hypothesis 
makes the earth of largest size at birth and of constantly dimin- 
ishing volume; the new regards the earth as beginning with a 
small nucleus and slowly growing by surface accretion, but 
with large reduction of volume by compression during and 
subsequent to the accretionary process. The old hypothesis 
involves the recognition of a primal, heated atmosphere and 
ocean consisting of the more volatile substances of the earth's' 
mass ; the new derives the present fluid envelopes from the 
earth's interior by a slow process of expulsion due to pressure 
and heat. 
The above brief contrast between the gaseous and the 
planetesimal hypotheses could be extended, but this will be 
suflficient to show how fundamentally opposed they are in their 
application to the origin of the globe. The bearing of the new 
hypothesis on a number of topics in physical geology will now 
be very briefly discussed. ■ ^ 
Origin of the Atmosphere. 
The nebular or gaseous hypothesis requires that the heated 
globe should be wrapped in the lighter and more volatile sub- 
stances. The atmosphere and hydrosphere are thereby made 
coeval with the globe itself. The fluid envelopes must have 
primarily contained, under high temperature, more or less 
material which was later given up to the lithosphere. Specu- 
lation has been indulged concerning the hypothetical de- 
posits formed by precipitation from the cooling waters ; but no 
rocks which can be referred to such genesis have ever been 
found. According to this conception the air and the sea are 
only the residue of the primeval envelopes, although it has been 
recognized that even under present geologic conditions there 
is interaction between the lithosphere and its fluid envelopes, 
with some exchange of material. For example, it seems impos- 
sible, under the view stated above, to escape the conclusion 
that all the carbon dioxide of the present atmosphere, along 
with that now stored in the rock strata, must have been origin- 
