542 r. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



existing without the sun. Thus Barrell, in his most recent estimates, , 

 places the age of the earth at a minimum of seven hundred and fifty 

 million years, with the possibility that it may be as much as one billion 

 five hundred million years ! Certainly there must be some readjustments 

 or mutual concessions between astronomers and geologists before these 

 widely discordant figures can be brought into harmony ! 



It is not only possible, but perhaps reasonable, to postulate a much 

 larger, more diffuse sun during the earlier stages of its existence, perhaps 

 a sun so large as to fill the orbit of the earth. With such a large-angle 

 sun the rays would fall nearly vertical on perhaps a whole hemisphere 

 at once. This would offer yan explanation for an equable, non-zonal dis- 

 tribution of climate and would make possible the growth of tropical or 

 subtropical vegetation within the polar circles. It would also provide an 

 explanation for tropical or subtropical glaciation, given a sufficient eleva- 

 tion of the land-mass. 



But it appears that there are certain astronomical objections that stand 

 in the way of accepting this in its entirety. Just what they are I can 

 not say, but I am informed by Dr. Abbot that he and Professor Moulton, 

 of Chicago, went over the subject some years ago and reached the con- 

 clusion that "not more than double the life of the sun as a source of 

 radiation capable of holding up the earth temperature could come that 

 way under the most favorable assumptions.^^ 



Heat from the earth itself. — We may now turn to the other assump- 

 tion, namely, a heat supply from the earth itself. The suggestion that the 

 sun was shut off by clouds and the heat came from within the earth itself 

 is, of course, not new; in fact, it follows as a natural corollary to one 

 of the most prominent of the theories advanced to account for the origin 

 of the earth. It is the essential basis of the views advocated by Dr. 

 Marsden Manson in his paper on the "Evolution of climates" and in 

 numerous subsequent publications, though it may be added that it doe- 

 not depend on the acceptance of the nebular hypothesis of earth origin. 



Setting aside for the moment the discussion of primitive earth tem- 

 peratures, we may consider the action that may reasonably be presumed 

 to result under this hypothesis of a dual heat supply. Granted an initial 

 surface temperature, from whatever source derived, that was higher than 

 that now existing, it can not be doubted that a larger percentage of the 

 water now existing in the seas, lakes, and rivers must then have existed 

 in the form of vapor. As it ascended, this vapor would be condensed 

 into clouds, forming a spheroidal cloud envelope surrounding the earth. 

 It also seems certain that under this hypothesis the sun must have exerted 



