18 EVOLUTION OF LIFE 1.9 



attached to these determinations is shown. Apparently the rate of 

 sedimentation has not been constant. 



It is conventional to postulate a series of crustal revolutions. The 

 extent of the movements has not been equal throughout and some of 

 them, more marked than others, were times of building of great 

 mountain chains such as the Alps or Andes (Fig. 4). There were also 

 many lesser rises and falls and changes of climate with shorter periods, 



Table I 



MAXIMUM THICKNESSES AND REVISED TIME-SCALE 



(ACCORDING TO HOLMES) 



THICKNESSES IN THOUSANDS OF FEET TIME SCALE IN MILLIONS OF YEARS 



such as those of about 40,000 years that we can detect in the later part 

 of the Pleistocene. Many modern geologists are sceptical about the 

 existence of any regularities or rhythms in these changes (see Herbert, 

 1952, and Gilluly, 1949). It is useful when trying to adjust the mind 

 to periods of 30 million years to remember the frequent changes of 

 level and climate that have occurred in the last 100,000 years. In 

 spite of all that we know about the history of the earth's surface, it is 

 necessary every time that we make statements about the influence of 

 presumed climatic changes on organic evolution to remember how 

 scanty our knowledge is. 



9.4. Classification of geological history 



The period isolated as 'Cambrian' by geologists lasted 100 million 

 years and almost certainly included several inundations, perhaps 

 three. The Ordovician lasted for 60 million years and included three 



