22 MEASURING TIME IN GEOLOGY 



this book not in facts alone but also in how they have been arrived 

 at, I think it is w^ell to thrash out properly the distinction between 

 the normal everyday geological method of relative dating, and the 

 means we now possess to measure geological time in absolute terms, 

 in years, or millions, or billions of years. 



RELATIVE DATING 



Again we must stress the fact that in dating the past, normal every- 

 day geology only supplies a relative or comparative age. The 

 methods used today essentially go back to the time of the English 

 surveyor and engineer William Smith (1769-1831). They are based 

 on two main tenets, viz. the Principle of Superposition on the one 

 hand, and Fauna Evolution in Geologic Time on the other. 



Principle of superposition 



The principle of superposition quite simply states that in any pile 

 of sedimentary rock, one bed was laid douTi after the other, each 

 bed being sedimented on top of the next underlying bed. The younger 

 bed is thus always superposed on the older, hence the name of this 

 principle. In this way a relative age can be assigned to a succession 

 of layers of rock, found, for instance, in a hill scarp or in a bore 

 hole. If it is possible to recognize the same succession in other hill 

 sides or bore holes, a correlation is established. And if in the latter 

 localities other beds are exposed, either older beds underneath the 

 known series, or younger beds on their top, then the local relative 

 time-table can even be extended. 



In some areas — for instance, in the London and Paris basins, 

 which among others formed a starting point for this type of age 

 determination — it is possible to follow such successions over quite 

 long distances along the hill sides. But any interruption of such a 

 series of exposures, for instance, a broad alluvial valley, a lake or an 

 ocean, limits the application of this method. It is often difficult or 

 impossible to tell which layer on one side of the gap is exactly to 

 be correlated with a given layer on the other side. This holds true, 

 for example, even for a narrow gap like the English Channel. The 

 white chalk cliffs of Dover and of Cap Blanc Nez, which look exactly 

 similar at first sight, already show many details in their lithological 

 succession which cannot be correlated across the intervening water 

 body. 



