COMPARISON OF HYPOTHESES 245 



ular planets were formed by the slow accretion or infalling of cold, 

 discrete bodies or particles ( u planetesimals "). 



The old hypothesis assumes an originally hot globe with shrinking 

 on account of cooling. The new regards the globe as originally and 

 always cold at the surface, and the interior heat as the product of grav- 

 itational 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 con- 

 stantly diminishing 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 en- 

 velopes 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 sufficient 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 substances. The 

 atmosphere and hydrosphere are thereby made coeval with the globe 

 itself. The fluid envelopes must have primarily contained under high 

 temperatures more or less material which was later given up to the 

 lithosphere. Speculation has been indulged concerning the hypothetical 

 deposits 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 geo- 

 logic conditions there is interaction between the lithosphere and its fluid 

 envelopes, with some exchange of material. For example, it seems im- 

 possible, 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 originally held in the volatile 

 envelopes. The difficulties in the way of harmonizing geological facts 

 with this conception have long been recognized by geologists, but have 

 been reserved for future solution. 



The new hypothesis claims that the substance of the atmosphere and 



