24 MEASURING TIME IN GEOLOGY 



implementation of the principle of organic evolution to the efforts 

 of the stratigrapher and the paleontologist, whose concern they are. 

 Suffice it to say that by painstaking use of these two guiding prin- 

 ciples, those of superposition and of organic evolution, an impressive 

 body of facts has been assembled. These facts permit dating the 

 geologic past in the relative time scale. All the commonly used 

 divisions of geologic time and also the main eras of Paleozoic, 

 Mesozoic and Cenozoic or Neozoic are based on evolutionary^ changes 

 of life, or, to be more exact, on the evolution of the fauna. Even the 

 names just mentioned, which divide geologic past into the eras of the 

 Early, Middle and the New Animal World — a division comparable 

 to Early History, Middle Ages and Newer History — even these 

 names indicate that these periods and their limits were defined by 

 the contemporary fauna, whilst the ancient flora plays only a sub- 

 ordinate part in relative geologic dating. 



Moreover, just as in human history, there are also geological pre- 

 historic and protohistoric periods, in which dating by organic evolu- 

 tion is impossible, in the same way as normal historic methods are 

 of no avail in human prehistory^ and protohistory. 



As we shall see, our quest as to the origin of life on earth will 

 eventually lead us into this earlier geological history. Before con- 

 tinuing, however, let me stress firmly once more that all normal 

 dating in geology is of a relative nature only. It tells us, for instance, 

 that beds of the Mesozoic are younger than beds of the Paleozoic. 

 It tells us that beds of the younger Cenozoic are very much younger 

 than beds of the early Paleozoic. But never is it possible from this 

 method of dating alone to state something like 'the Mesozoic began 

 umpteen million years ago'. Any statement giving a definite number 

 of years is based on the methods of absolute dating, and not on the 

 normal geologic way of relative dating of the past. 



Relative age of sediments and igneous rocks 



One more fact has to be put forward in relation to this relative 

 dating of the geologic past: it can only be applied directly to sedi- 

 ments. Igneous rocks, formed within the crust through the gradual 

 cooling and solidifying of molten magma, do not follow the principle 

 of superposition. Molten magma tends to break through the crust and 



