16 EVOLUTION OF LIFE 1.9 



changed; thus rather sharp breaks appear in the series of fossils. The 

 occurrence of these breaks has been used by geologists to define the 

 major geological periods, which thus correspond to cycles of elevation 

 and depression of the continents. By comparing the fossils contained 

 in the rocks major geological periods have been recognized in various 

 parts of the world. The times of submergence and emergence differ 

 from region to region, however, and no very close detailed comparison 

 is possible. It is easy to forget that climates and land levels do not 

 always change in the same direction in different parts of the world. 



9.3. Geological time 



Until recently most geologists assumed that there was a regular 

 cycle of raising and lowering (diastrophism) and that comparable 

 periods could be recognized everywhere. It is now widely doubted 

 whether there has been any such 'pulse of the earth'. The rock series 

 are not the same in all the continents. For example, in South Africa 

 three long series, known as Cape, Karoo, and Cretaceous formations, 

 occupy the time covered in Europe by the many elevations and depres- 

 sions between Silurian and Cretaceous times. Probably the conditions 

 under which rocks were formed have remained about the same 

 throughout geological time but have been interfered with by periods 

 of elevation, depression, and folding that are peculiar to each region. 



The study of fossils often establishes the order in which the rocks 

 were laid down, but other methods have to be used to discover the 

 period of time covered by each stage. This is especially important to 

 the biologist, who wants to know the rate at which animals or plants 

 have evolved. Reliable knowledge of the ages of the rocks has only 

 begun to accumulate since the discovery of radioactivity. Uranium 

 and thorium disintegrate, producing lead, at rates that are unaffected 

 by any known conditions. The age of any rock since its deposition can 

 therefore be calculated if we can estimate the amount of breakdown 

 products of these elements present in it. The lead present in a rock is 

 often not all derived from the uranium and thorium there, but separa- 

 tion of the lead isotopes enables those of radioactive origin to be 

 estimated, and the age of the deposit can then be determined, assum- 

 ing that the breakdown of uranium to lead began when the rock was 

 crystallized in its present position. Other methods of estimating the 

 ages of rocks from isotope ratios have been developed. Especially 

 promising is the determination of the ages of the deposition of sedi- 

 mentary rocks from the ratio of A 40 /K 40 and Sr 87 /Rb 87 in deposits 

 formed by erosion of micas or granites. 



