PHYSIOGRAPHIC PROBLEMS OF TO-DAY 633 



The growth of physiography up to the present time has been 

 largely influenced by the far-reaching ideas of Laplace and others in 

 reference to the nebular origin of the solar system. In all of the 

 questions respecting secular changes of land areas in reference to the 

 surface level of the ocean, the origin of corrugated and of block 

 mountains, the fundamental nature of volcanoes, etc., the controlling 

 idea has been that the earth has cooled from a state of fusion, and is 

 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 com- 

 pression and consequent heating of the growing globe by reason of 

 gravitational 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 sur- 

 face. 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 experi- 

 ence, it is evident that the planetesimal hypothesis necessitates a re- 

 vision, 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 hy- 

 pothesis has been tried and to a marked extent found wanting, and 

 that the new conception of the mode of origin of the earth demands 

 consideration, not only as affecting a large group of basement princi- 

 ples of interest to the physiographer, but with the view of testing the 

 planetesimal 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 in- 

 crease or diminution; the results of gravitational contraction in refer- 



