[ 23 ] 



III. 



AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 

 By J. JOLY, M.A., B.A.I., D.Sc, F.R.S., F.G.S., M.R.LA., 



Honorary Secretary of the Eoyal Dublin Society ; 

 Professor of Geology and Mineralogy in the University of Dublin. 



[Bead May 17, 1899.] 



Introduction. 



The extremes to which, in the time of Lyell, the principles of Uniformitarianism 

 were carried did much to injure a doctrine which, properly restricted, defines 

 the only scientific attitude open to the Geologist in dealing with the past and the 

 future. 



Rightly defined this doctrine is no other than that held and lived up to by 

 every scientific man. It asserts that we may justly prolong into the past and 

 future the activities of to-day, till sufficient reason be shown to interrupt them by 

 catastrophe or change. The onus of examining into the " sufficient reason " rests 

 with the disciple of Uniformitarianism. It is, in fact, his business to seek and 

 define the limitations in time of the actions he is familiar with. 



He differs from the Catastrophist or Convulsiouist in the stringency with which 

 he defines and examines the reasons for postulating such changes and catastrophies, 

 and it may be said the reluctance with which he resorts to such modes of 

 explanation. If existing operations, when extended into the past, are not in 

 discord with probabilities, he prefers the existing operations to alternative ones, 

 even if the latter in themselves involve nothing im^^robable. 



The assumption of uniformity of present activities enters into many attempts 

 to estimate the Age of the Earth dated from the beginning of those changes which 

 may be referred to the action of water upon the face of an igneous lithosphere. 

 Such attempts, broadly speaking, deal with the fact that a lithosjjhere, cooling 

 from fusion, and then subjected to aqueous solution, is molecularly unstable in 

 presence of the latter agency. Nor can final stability be attained till all molecular 

 ties are remade in the common solvent, and retained under the conditions of 

 their formation : in other words, till complete solution has been efPected, and all 

 is immersed in the common solvent. It is possible that so long as subsidence and 

 elevation are possible, tides exist, and evaporation progresses, this state cannot be 

 attained. To-day we find ourselves in the midst of these cycles. "We perceive 

 soils formed under subaerial actions, partly from former igneous rocks, partly from 



TRANS. EOT. DUB. SOC, N.S. VOL. TU., PAET UI. Tf 



