Ill] THE GEOLOGICAL RECORD 41 



such rocks the remains of calcareous seaweeds are 

 frequently recognisable and occasionally, as in the 

 English chalk, fragments of wood testify to transport 

 from a distant land. 



While there is little difficulty in explaining the 

 nature of much of the earth's crust, in several parts of 

 the world the strata are totally unfossiliferous and 

 closely simulate crystalline rocks. In many cases 

 it is believed that such strata represent ancient 

 sediments which in the course of ages have been 

 reduced by metamorphic agencies to a condition 

 which has obscured or entirely obliterated all traces 

 of their pristine state. 



Since the pioneer work of William Smith, who 

 in the early days of the nineteenth century first 

 realised the importance of fossils as aids to the 

 determination of relative age, geologists have devoted 

 themselves to the task of correlating the sedimentary 

 rocks of the world, using as criteria the order of 

 superposition of the strata and the nature of their 

 organic remains. The result has been to classify 

 portions of the earth's crust into periods or chapters, 

 which together constitute a record of geological 

 evolution as complete as it is possible to obtain from 

 the available data. The accompanying table shows 

 the order of sequence of the epochs, which stand for 

 terms of years of a magnitude beyond our powers to 

 grasp. 



