INTRODUCTION. 
35 
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 fauna 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 United States, 
that the lines of demarcation between subordinate groups, and the 
line of separation between systems are equally strong, and 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 into systems have been made dependent on the 
imperfection rather than the perfection of the sequence. 
