46 INTRODUCTION. 



divisions of such formations with anything approaching to 

 absolute precision. Kegarded as a whole, however, the Car- 

 boniferous formation of America is the geological equivalent 

 of the Carboniferous formation of Europe, and .both belong 

 to what geologists understand as the " Carboniferous period." 

 As the same is true of all the great formations, in all parts 

 of the world, it is clear that the principal advantage of the 

 use of such a term as " homotaxis " is simply that we thereby 

 avoid the employment of a word which common usage would 

 wrongly interpret ; and it is quite certain that we cannot 

 abolish the idea of geological " contemporaneity," as demon- 

 strated by the presence of identical or representative species 

 of fossils; nor can we refuse to admit that formations con- 

 taining such fossils, however far removed from one another 

 in point of distance, must have been laid down within the 

 limits of the same great " period " in the history of our 

 earth. 



We are now in a position very briefly to discuss the 

 question of what may be called ."geological continuity." 

 It has already been stated that the entire series of Fossil- 

 iferous or Sedimentary rocks may be naturally divided into 

 a certain number of definite rock-groups or " formations," 

 each of which is characterised by the possession of a peculiar 

 and characteristic assemblage of fossils, constituting, or rather 

 representing, the " life " of the " period " in which the for- 

 mation was deposited. The older geologists held, what 

 probably every one would be tempted to think at first, 

 that the close of each formation was characterised by a 

 general destruction of the forms of life of the period, and 

 that the commencement of each new formation was accom- 

 panied by the creation of a number of new animals and 

 plants, destined to figure as the characteristic fossils of the 

 same. This theory, however, not only invokes forces and 

 processes which it can in no way account for, but overlooks 

 the fact that most of the great formations are separated by 

 lapses of time, unrepresented perhaps by any deposition of 

 rock, or represented only in some particular area, and yet, 

 perhaps, as great as, or greater than, the whole time occupied 

 in the production of the formation itself. 



