GEOLOGY 131 



times it has been extended into the realm of cosmogony in 

 such a way as to give it geological significance. This is in 

 the Planetesimal Hypothesis of Professor ChamberHn of the 

 University of Chicago. Professor Reese last week told you 

 something of this hypothesis. It is not my intention to dis- 

 cuss its astronomical significance, or to attempt to explain the 

 astronomical difficulties that it may encounter. It is essen- 

 tially a theory of cosmogony, but it is possible by its help 

 to explain many facts of earth development in geological time 

 that were much more difficult to explain under the hypothesis 

 of Laplace. You will recall from Professor Reese's lecture 

 that the hypothesis supposes that the earth has grown to its 

 existing size by the accumulation of the sweepings of the 

 heavens, so to speak. It has gathered up and incorporated 

 with itself — digested — a great number of small bodies called 

 planetesimals that it found revolving in space within the range 

 of its grasp. These bodies, as they float in space, are cold. 

 Their impact with each other or with the earth develops heat. 

 The earth's heat, therefore, so long as this accumulation was 

 going on, was increasing rather than decreasing. 



Under the nebular hypothesis the earth began, after the 

 material was once gathered into one body, as a highly heated 

 gaseous body at first, changing later to liquid on cooling, and 

 still later to a solid crust through the freezing of the outside 

 due to heat radiation from the surface. According to this 

 hypothesis, the geological history of the earth has been a his- 

 tory of gradually declining temperature. The early stages 

 in its history were characterized by high temperatures, dense 

 cloudy atmosphere, and abundant rainfall. The hypothesis 

 was developed by means of mathematical reasoning based on 

 the facts of astronomy and not on the facts of geology. The 

 latter had not been accumulated at the time of the promul- 

 gation of the theory, and by the time they had accumulated 

 the theory was so universally accepted that the geological 

 facts that did not harmonize with it were silently passed over. 



