38 INTRODUCTION. 



tially compact and homogeneous fauna from base to summit 

 were deposited with comparative rapidity. Upon this view, 

 a formation like the Lias is one formed by a process of very 

 slow and intermittent sedimentation, the life -zones being 

 separated by intervals, during which sedimentation must 

 have been at a stand-still, but which were long enough to 

 allow of more or less considerable biological changes, some 

 forms dying out, or becoming modified, while other new ones 

 came in. Upon this view, further, a formation like the Lias, 

 though of comparatively small vertical extent, may represent 

 as long a period of time as the whole of such a great forma- 

 tion as the Carboniferous, which appears to have been formed 

 under conditions of comparatively rapid sedimentation. 



CONTEMPORANEITY OF STRATA. 



When groups of beds in different parts of the earth's sur- 

 face, however widely separated from one another, contain the 

 same fossils, or rather an assemblage of fossils in which many 

 identical forms occur, they are ordinarily said to be " contem- 

 poraneous ; " that is to say, they are ordinarily supposed to 

 belong to the same geological period, and to have been formed 

 at the same time in the history of the earth. They would 

 therefore be unhesitatingly regarded as "geological equiva- 

 lents," and would be classed as Silurian, Devonian, Carbon- 

 iferous, and so on. It is to be remembered, however, that it 

 is not necessary, to establish such a degree of equivalency 

 between widely separated groups of strata, that the fossils of 

 each should be to any great extent specifically identical. It 

 is sufficient that, whilst some few species are identical in 

 both, the majority of the fossils should be " representative 

 forms," or, in other words, nearly allied species. It will be 

 shown, however, that groups of strata widely removed from 

 one another in point of distance can only exceptionally be 

 " contemporaneous," in the strict sense of this term. On the 

 contrary, in so far as we can judge from the known facts of 

 the present distribution of living beings, the occurrence of 

 exactly the same fossils in beds far removed from one another 

 is primd facie evidence that the strata are not exactly con- 



