PALAEONTOLOGY AND GEOLOGY 



of creation signify six periods of time in the evolu- 

 tion of the world, which has slowly cooled from a 

 condition of incandescence to its present temperature, 

 gradually changing its inhabitants to adapt them- 

 selves to new conditions; and the world will continue 

 to cool until animal life can no longer be supported. 

 As Buff on relied on the energy of universal gravita- 

 tion and on heat as the causes of secular changes, his 

 first effort was to calculate the duration of the earth. 

 For this purpose he carried on many experiments to 

 determine the rate of cooling of various minerals and 

 rocks. In particular, he heated great iron globes to a 

 very high temperature in order to estimate from their 

 rate of cooling how long the earth would require to 

 reach its present temperature. His conclusion was 

 that it needed about 75,000 years to change from a 

 molten mass to its present state. It is not necessary to 

 follow in detail his elaborate guesses as to the dura- 

 tion of each of his epochs, or geological ages, or to 

 enumerate his succession of flora and fauna. The im- 

 portant fact is that this scheme is one quite different 

 from the fanciful accounts of classical or renaissance 

 authors. It is modern. He derived his periods from 

 the thickness and character of rock strata and the 

 physical laws of cooling of the earth. He freely em- 

 ployed long periods of time with the forces of gravi- 

 tation and heat as the only active agents, and he di- 

 vided his epochs by the corresponding prevalence of 



1: 141 : 



