FUNDAMENTAL PROBLEMS OF GEOLOGY 263 



years, and not more than 350 or 400 miles in the enormous period 

 of 600,000,000 years. The inference inevitably follows that all the 

 movements and deformations of the crust of the earth, due to cooling, 

 are confined to very shallow depths relative to the radius of the 

 earth, and that the greater interior mass of the geoid has not ap- 

 preciably participated in thermal effects. This conclusion, if true, is 

 radical. Assumed to be true, it has profoundly influenced geolog- 

 ical inquiries and interpretations. 



With the growth of evidence that the temperature of solidification 

 rises with pressure, there has grown the hypothesis that the earth 

 solidified first at the center, because of the high pressure there resi- 

 dent, and later congealed at higher and higher horizons in succes- 

 sion until the solidification reached the surface. This view has come 

 to replace the older view to a large extent, in competent opinion, but 

 the necessary corollary that the initial temperature of the solidified 

 globe would be high at the center and grade thence to the surface, 

 though recognized, has not replaced in an equal degree, in dynamical 

 studies, the older view of an essentially uniform thermal distribu- 

 tion. As a result, we have no system of dynamical doctrine worked 

 out consistently on this later hypothesis. 



In attempting some years ago to apply the kinetic theory of gases 

 to atmospheric problems, I became impressed with the weakness of 

 the gaseous hypothesis of the earliest stages of the earth's evolution, 

 and subsequent studies of the relations of mass and momenta, with 

 the essential aid of Dr. Moulton, led to still graver doubts as to the 

 tenability of the Laplacian hypothesis, on which both of the preced- 

 ing conceptions of the origin and distribution of internal heat are 

 based. This led to studies upon alternative hypotheses. Among 

 these is the conception that the earth, instead of descending from a 

 gaseous spheroid, may have been built up by the gradual ingather- 

 ing of its material from a scattered meteoroidal or planetesimal con- 

 dition. If the infall were sufficiently rapid a molten, and even a 

 gaseous, condition would result, but if the infall were slow the sur- 

 face heat arising from impact might be radiated away practically as 

 fast as generated, and the accretion might proceed with compara- 

 tively cool surface temperatures. The source of the obviously high 

 internal temperatures of the earth then arose as a question crucial 

 to the hypothesis. The suggestion that it might be due to gravi- 

 tative compression arising from the earth's increasing mass was 

 submitted to preliminary computation with favorable results. This 

 therefore appeared to give a third working hypothesis of the origin 



