450 Rev. Maxwell S. Close — Extent of Geological Time. 



V. — The Extent of Geological Time.^ 



By the Eev. Maxwell H. Close, M.A., P.G.S. ; 



President of the Eoyal Geological Society of Ireland. 



THE question of the geological age of the earth has been of late 

 very prominently before the minds both of geologists and 

 physicists. There is no occasion to take up time by giving a sketch 

 of the late history of the discussion. I beg leave simply to point 

 out some considerations which seem to lessen considerably the 

 weight of the physical objections to the great extent of geological 

 time. Let us observe before proceeding further that we do not wish 

 to avoid wholesome restriction of geological time. It seems to me 

 that it adds greatly to the interest of geological investigation to 

 know that we have not a wilderness of possibility before us as to 

 the length of time and the consequent deliberateness of geological 

 operations. If we feel inclined to complain that Sir William 

 Thomson is rather severe upon us, let us reflect that in this matter 

 he is generosity itself compared to a certain distinguished collabo- 

 rateur of his, and moreover that if he lays a burden upon our 

 shoulders, he supplies us with a good staff to help us in carrying it. 

 He points out that while the earth was hotter than now, its energy 

 was greater, and the rate of working of geological agents greater. 

 This would lessen the need for greatly extended time. We should 

 admit that there were, probably, two circumstances which would 

 make that increased energy less readily available, and diminish the 

 rate of its expenditure. Those are the cloud-covering that must 

 have enveloped the earth to interfere with radiation, and the greater 

 extent of sea and smaller extent of land, which is supposed to have 

 existed in the earlier geological ages, which would lessen the 

 amount of denudation by diminishing its area. The first physical 

 objection that we shall now mention is that drawn from the 

 supposed rate of cooling of our globe, and the time that would be 

 necessary for it to reach its present thermal condition. Sir William 

 contemplates granting the geologists 90,000,000 years, though he 

 may have to allow them only 50,000,000. In making the calcula- 

 tion it is necessary to make two estimates and two assumptions, on 

 which to found the calculation. The first estimated quantity is the 

 increase of temperature per unit of depth in the crust of the earth, 

 which he takes at 1° Fahr. for 54 feet. The second is the con- 

 ductivity of the body of the earth, or at least of its less inward part. 

 Mr. Mellard Eeade has shown the great uncertainty there is as to 

 both of these. The two assumptions are the approximately equable 

 distribution of the temperature of the globe at its first solidification, 

 and the constant difference of temperature between the surface and 

 the interior, of more than 7,000° Fahr. 



The next argument for the restriction of geological time which we 

 shall consider is that the sun cannot be imagined capable of keeping 

 by its radiation the earth's surface in a state fit for the support of 



1 Being a paper read before the British Association (Section C), Dublin, 

 August 21, 1878. 



