AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



By J. JoLY, M. A., D. Sc, F. R. S. 



INTRODUCTION. 



The extremes to which, in the time of Lyell, the principles of Uni- 

 formitarianism 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. 



Rightl}" 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 suflicient reason 

 be shown to interrupt them by catastrophe or change. The onus of 

 examining into the "suflicient reason" rests with the disciple of uni- 

 formitarianism. It is, in fact, his business to seek and define the limi- 

 tations in time of the actions he is familiar with. 



He differs from the catastrophist or convulsionist 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 improbable. 



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, broadl}^ speaking, deal 

 with the fact that a lithosphere, 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 effected 

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



^ Read May 17, 1899, by J. Joly, honorary secretary of the Royal DubHu Society, 

 professor of geology and mineralogy in the University of Dublin. Reprinted from 

 Scientific Transactions of the Royal Dublin Society. New Series, Vol. VII, Part 

 III, 1899. 



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