GENERAL PRINCIPLES 7 



of England was, however, the first to recognize the fundamental 

 significance of fossils for determining the relative ages of stratified 

 rocks. His announcements, based upon much careful detailed 

 work, were made in the latter part of the eighteenth century and 

 the early part of the nineteenth century. He has been called by 

 the English the " Father of Historical Geology." 



1. Earth chronology. In any given region the best way to 

 learn the relative ages of the stratified rocks is to determine their 

 "order of superposition," the general assumption being that the 

 older strata underlie the younger because the underlying sediments 

 must have been first deposited. While this is a fundamental 

 method, it is very limited in its application when used alone in 

 regard to the construction of the whole earth's history. The suc- 

 cession of strata seen in any one locality or region represents only 

 a small part of the earth's entire series and this, taken in connec- 

 tion with the fact that the lithologic character of strata of the same 

 age frequently changes, makes it clear that " order of superposi- 

 tion" alone will not suffice to determine the relative ages of sedi- 

 mentary rocks on a single continent or even large portion of a 

 continent, not to mention the utter inadequacy of the method 

 when applied to comparing the relative ages of strata of different 

 continents. 



" Order of superposition," however, when used in connection 

 with the fossil content of the strata, furnishes us with the method 

 of determining earth chronology. "Life, since its introduction on 

 the globe, has gone on advancing, diversifying, and continually 

 rising to higher and higher planes . . . Accepting, then, the un- 

 doubted fact of the universal change in the character of the organic 

 beings which have successively lived upon the earth, it follows that 

 rocks which have been formed in widely separated periods of 

 time will contain markedly different fossils, while those which are 

 laid down more or less contemporaneously will have similar fossils. 

 This principle enables us to compare and correlate rocks from all 

 the continents and, in a general way, to arrange the events of the 

 earth's history in chronological order ... A geological chronology 

 is constructed by carefully determining, first of all, the order of 

 superposition of the stratified rocks, and next by learning the fossils 

 characteristic of each group of strata . . . The order of succes- 

 sion among the fossils is determined from the order of super- 

 position of the strata in which they occur. When that succession 



