666 Transactions. — Geology. 



Art. LXX. — Denudation as a Factor of Geological Time. 

 By H. Hill, B.A., F.G.S. 



[Read before the Haioke's Bay Pliilosophical Institute, 10th June, J893.] 



The many discussions among geologists and physicists which 

 have taken place from time to time as to the age of the earth 

 have bi-ought under review wide diversities of opinion. The 

 mind is lost in wonder as we go back and back from age 

 to epoch, from epoch to period, and from period to era in the 

 order of geological time as illustrated by stratigraphy, or the 

 sequence of the rocks ; but these chronological terms only 

 represent one aspect of activity in the full history of the 

 earth. Time was when the conditions of the earth were such 

 that life, as we know it to-day, could not exist ; when the 

 waters which now cover its surface could not have remained 

 as a liquid ; and when from pole to pole the rocks were 

 heated beyond the possibility of sustaining life. The geologist 

 is unable to say how long the earth remained in the condition 

 of inability to sustain organic life, for, reasoning from the 

 known, his aim is to estimate time by the changes such as he 

 knows nmst have taken place since life first became possible 

 on the earth, and when diflerentiations were the fewest con- 

 sonant with what is understood as the maintenance of the 

 simplest organisms. The physicist, on the other hand, deals 

 with the history of the earth from an altogether different 

 standpoint. He views it as something to be interpreted only 

 by the application of certain laws to physical or assumed 

 physical conditions, his conclusions being based — first, upon the 

 assumed original temperature of the earth, and its annual rate 

 of cooling; second, upon the loss of energy of the earth's 

 rotation in virtue of the moon's attraction, as evidenced by 

 the tides ; and third, upon the origin and age of the sun as a 

 dispenser of heat. On the supposed original temperature of 

 the earth and its annual loss of heat by radiation, estimates 

 have been made that the globe could not have become con- 

 solidated less than twenty millions of years ago, nor more 

 than four hundred millions ago ; for, had the latter been the 

 case, there would have been no alteration of temperature when 

 descending mto the earth, such as is everywhere experienced 

 by miners when sinking or boring through the earth's crust. 

 As to the loss of energy of the earth's rotation owing to the 

 friction of the tide-wave. Sir William Thompson contends 

 that, had the earth become solid some ten thousand million 

 years ago, or, indeed, any period beyond one hundred million 

 years, the polar flattening would have been greater than now, 



