INTRODUCTION. 89 



deposits may have been synchronous ; or whether indeed farther inves- 

 tigation along the line of the junction of these formations may not 

 disclose new relations; for though the great mass of a recognized group 

 of strata will furnish the prominent data for the more comprehensive 

 conclusions, it is only by a careful search along the line of junction of 

 the successive groups that we become acquainted with those conditions 

 of change which ushered in the new era. It is by these investigations 

 that we acquire numerous facts of apparently minor importance in 

 themselves, but which lie at the foundation of all our conclusions 

 regarding the true relations of the successive parts in the great geo- 

 logical sequence. 



The most striking contrast between the fauila of the Lower and 

 Upper Helderberg groups is in the abundance of large corals and the 

 remains of fishes in the latter, while in the former we find few large 

 corals and no remains of fishes. And although in the general expression 

 of the brachiopoda of the two periods there may be no very marked 

 differences, yet the evident relations of the Lower Helderberg fauna 

 to the Niagara fauna will be seen at every step of comparison, and 

 shown in the illustrations and descriptions in the following pages; If 

 therefore similarity of physical conditions and similarity of fauna are 

 to govern us in determining the relations of formations, then the Lower 

 Helderberg group should be united with the Niagara group in one great 

 system. 



In regard to these questions, however, I have long since expressed 

 the opinion, founded upon extensive observation in the IJriited States, 

 that the lines of demarcation between subordinate groups, and the 

 line of separation between systems are equally strong, atid that the 

 whole series may be regarded as a succession of minor groups ; that 

 the strong lines of division are almost always due to the absence of 

 some formation, which if present would show a gradation to the next ; 

 and these subdivisions itito systems have been made dependent on the 

 imperfection rather than the perfection of the sequence. 



