PHYSIOGRAPHIC PROBLEMS OF TODAY 531 



still shrinking on account of the dissipation into space of its internal 

 heat. 



With the recent presentation of the planetesimal hypothesis by- 

 Professor Chamberlin, a radically different point of view is furnished 

 from which to study the internal condition of the earth. The new 

 hypothesis — which has for its main thesis the building of a planet by 

 the gathering together of cold, rigid, meteoric bodies, and the compres- 

 sion and consequent heating of the growing globe by reason of gravita- 

 tional contraction — is suggestive, and seems so well grounded on facts 

 and demonstrated physical and chemical laws that it bids fair not 

 only to revolutionize geology, but to necessitate profound changes in 

 methods of study respecting the larger features of the earth's surface. 

 One of the several considerations which make the planetesimal 

 hypothesis appeal forcibly to the inquiring mind is that it employs an 

 agency now in operation, namely, the process of earth-growth through 

 the incoming of meteoric bodies from space; and for this reason is 

 welcomed by uniformitarians, since it is in harmony with their under- 

 standing of a fundamental law of nature. 



In many, if not all, questions respecting the origin of the atmosphere, 

 the ocean, continents, mountains and volcanoes, and the secular, and to 

 a marked extent in certain instances, the daily changes they experience, 

 it is evident that the planetesimal hypothesis necessitates a revision, 

 or at least a review, of some of the fundamental conceptions held by 

 physiographers. The objection will perhaps be advanced that to 

 make such a radical change of front on the basis of a young and as yet 

 untried hypothesis is not wise. The reply is that the older hypothesis 

 has been tried and to a marked extent found wanting, and that the 

 new conception of the mode of origin of the earth demands considera- 

 tion, not only as affecting a large group of basement principles of 

 interest to the physiographer, but with the view of testing the planetesi- 

 mal hypothesis itself by physiographic standards. 



The problems interlocked with the mode of origin of the earth, in 

 which the physiographer shares an interest with the geologist, are the 

 rate at which the earth's mass is now being increased owing to the 

 ingathering of planetesimal, and the chemical and physical and per- 

 haps life conditions of the incoming bodies; the temperature of the 

 earth's interior, and the surface changes to be expected from its 



