244 H. L. FAIRCHILD— GEOLOGY UNDER PLANETESIMAL HYPOTHESIS 



matical form by Laplace in a tentative way. It has been called the 

 grandest conception of the human mind, and, for the eighteenth century, 

 it might have been regarded as good science. It fails, however, to meet 

 the requirements of a modern scientific hypothesis because it does not 

 explain all the facts. But in the absence of an}^ rival theory it has held 

 possession of the field, and has been more generall} 7 accepted on insuffi- 

 cient basis than any other conception in scientific philosophy. The time 

 has come for judgment and probable condemnation of the old hypothesis. 

 The formulation of a new and better hypothesis by an honored Fellow 

 and former President of this Society will be recognized in the future as 

 opening a new epoch in earth science and as one of the glories of American 

 geology. This is true even if the new hypothesis should not wholly stand. 



The critical application of accepted principles of physics to the nebular 

 hypothesis, by Professor Chamberlin, has revealed its weakness. The 

 planetesimal hypothesis which he has formulated as a substitute seems 

 to explain much better both the astronomical and geological phenomena. 

 It may not be immediately and universally accepted, as it destroys the 

 present foundation of many geophysical theories and because the leaders 

 in science are committed to the old ideas ; and in all its claims it may 

 not be true, but its main postulate, that the globe was formed by accretion 

 of cold matter, will probably stand. 



The purpose of this writing is not to make an extended argument for 

 the new hypothesis * nor to discuss its relations to celestial physics, nor 

 even to the solar system, but to indicate its bearings on several problems 

 in geology and very briefly to show how these problems are simplified 

 b} 7 the new conception. It is predicted that under the stimulus of the 

 new thought so many changes will be made in our views of geologic pro- 

 cesses that the science of geology will be rejuvenated, as its theoretic or 

 philosophic advance has been seriously retarded by its dependence on a 

 false conception of earth genesis. 



Comparison of the Nebular and Planetesimal Hypotheses 



The old hypothesis assumes the existence of a mass of incandescent 

 vapor, with or without a nucleus, which by condensation and rotation 

 was differentiated into successive rings, the latter being eventually gath- 

 ered into the planets while still retaining intense heat. From this post- 

 ulate there necessarily follows the conception of a cooling earth, and 

 hypogeic geology has been founded on the idea of crustal solidification 

 on a molten globe. The new hypothesis holds that the disseminated 

 planet-forming matter had lost its heat while yet existing in the loose 

 form, as rings or zones or wisps of the parent nebula, and that the glob- 



* The full exposition of this hypothesis will be found in a new text-book of geology by T. C 

 Chamberlin and R. D. Salisbury. 



