Ese 
III. 
AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 
IB do MOI, Wl/hoy Ied/Nclle, IDS, Tol, JAKCHSh, Milley) sly, 
Honorary Secretary of the Royal Dublin Society ; 
Professor of Geology and Mineralogy in the University of Dublin. 
[Read May 17, 1899. | 
Introduction. 
Tue 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 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 1f the latter in themselves involve nothing improbable. 
The assumption of uniformity of present activities enters into many attempts 
to estimate the Age of the Harth 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 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 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, ROY. DUB. S80C., N.S. VOL. VII., PART III, F 
