1.9 



GEOLOGICAL TIME 



17 



The time at which the crust of the earth assumed its present form 

 is now thought to have been 4,500 million years ago (Holmes, 1959) 

 but the rocks laid down during the greater part of this long period 

 contain no undoubted animal or plant remains. Cambrian rocks, 

 when fossils become readily discernible, were laid down about 600 

 million years ago. 



25 60 

 11 40 70 



MILLIONS OF YEARS 

 135 180 225 270 305 350 400 440 



500 



600 



21 

 42 



68 



98 

 110 



£ 161 



u. 205 



CEpliocene 

 sTl i i i 

 ^j- MIOCENE 



"IjJ-o'ugocene 

 ~3JJeocene 



o 



235 

 254 

 2 74 

 300 



O , 



38 



372 



412 



452 



PRECAMBRIAN 



_i I I I I I I i l_ 



100 



200 300 400 



MILLIONS OF YEARS 



500 



■ ■ ' ■ 



600 



i i i i . 





 50 

 100 

 150 

 200 

 250 

 300 

 350 

 400 

 450 



700 



Fig. 3 shows the maximum thickness of sediment in each period plotted against estimates 

 of the absolute date. The error attached to these determinations is shown by the 

 marginal lines. Apparently the rate of sedimentation has not been constant (modified 



after Holmes). 



Classical geology is based mainly on studies in Europe and North 

 America. Although a terminology based on absolute time is beginning 

 to emerge, it is still necessary to use that based mainly on stratigraphic 

 studies, begun by William Smith in the British Isles early in the nine- 

 teenth century. In this system, the time since the Cambrian is divided 

 into eleven major periods, but several of these were double or triple 

 periods of advance and retreat of the sea. Even the most carefully 

 compiled radioactivity data are not yet adequate to provide us with 

 definite estimates of the durations of the periods, though there is 

 agreement on a total period of about 600 million years since the 

 Cambrian. Fig. 3 shows the maximum thickness of sediment in each 

 period plotted against estimates of the absolute dates. The error 



