﻿THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE II. VOL. IV. 



No. I— JANUARY, 1877. 



I. — On Evolution in Gteologt. 



By W. J. SoLLAS, B.A., F.G.S., 

 Lecturer on Geology, Cambridge University Extension. 



THAT the energy of the earth and the sun is a continually diminish- 

 ing quantity, and must at the beginning of geologic history 

 have been far in excess of its present amount, are propositions that 

 few at the present day would be found to deny ; but the exact influence 

 of this greater quantity of energy on geologic changes has not, I 

 believe, been hitherto fully discussed.^ 



The Sun. — The Sun is a large and exceedingly hot star, cooling 

 slowly in space, and it is because it is so large and hot that it has 

 been able to supply our world and all the other members of its 

 system with so much heat for so long a time. It is daily losing 

 heat, and it is the heat thus lost by the sun, and gained in part by 

 us, that supplies the energy requisite for the work of most denuding 

 agents. 



If the sun then be a cooling body, its store of energy must have 

 been greater yesterday than it is to-day, by exactly the equivalent 

 of the amount of heat it has radiated since then ; and so of all 

 previous yesterdays : so that if we go back far enough in time, we 

 shall reach a period when the store of energy in the sun was too 

 great to joermit of its existence in its present state. This period, as 

 calculated by Sir William Thomson, is placed as far back as from 

 100 to 500 millions of years ago.^ But it is a law of cooling bodies 

 that they cool the quicker the higher their temperature is above that 

 of the surrounding medium. The space in which all worlds exist, 

 the medium which surrounds our sun, appears to have no sensible 



' The views contaiQed in this paper, the sources of some of which will be obvious 

 were first expressed before the Halifax Philosophical Society, in a lecture which I 

 delivered in 1874, and afterwards in a paper read before the Leeds Geoloo-ists' 

 Association. They are now printed in a condensed form and rather as suggestions 

 than anything else. 



* The increased amount of energy in the sun does not imply a corresponding 

 elevation of temperature; to a great extent such additional energy was poten- 

 tial, but from all we know it seems most probable that an increase in potential 

 would be accompanied in this case by an increase in kinetic energy, and so we may 

 safely assume that the temperature of the sun rises in an ascending curve as we 

 proceed backwards in time. The more rapid conversion of potential into kinetic 

 energy, which probably occurred at intervals, would produce fluctuations in 

 temperature, and so the temperature curve in the sun's history is probably not a 

 simple Kne, but varied by numerous minor xindulations. 



DECADE II. VOL. IT. — NO. I. 1 



