INTRODUCTION. 
87 
of its fauna is manifested at intervals in a few feet of limestone lying 
below these siliceous beds. 
The northern outcrops of the Keokuk, Warsaw, and St. Louis lime* 
stones are also farther north than that of the Kaskaskia, while in the 
south they have so far thinned out as to form no important feature ; 
nevertheless the fauna is in many places partially represented, and it 
is probable that more careful examinations may develop a clearer dis^- 
tinction among the members of the series, when their relations and 
their fauna in more northerly localities shall have been fully studied. 
Thus, at the south, it would have been impossible to establish the 
sequence; nor could it well have been established by investigations 
progressing from south to north. It is necessary to trace them from 
their northern outcrops southward, in order to have a full appreciation 
of these different members and their relations, to become acquainted 
with each rock in its order, and to learn its distinctive fauna. To an 
investigator of the Carboniferous limestone in Tennessee and Alabama, 
the mass is essentially a unit, and the condition of the lower fauna is 
not such as to be readily suggestive of subdivision. But from a northern 
point of view, each period is seen to have had a distinctive accumula¬ 
tion ; each one is marked by its fauna, indicating a change in the 
conditions of the bottom or depth of the ocean; each successive period, 
while having the greatest development in the north, has witnessed 
a recession of the Carboniferous sea towards the south. This gradual 
recession has continued to the period of the Kaskaskia limestone, 
while the maximum development of this formation lies in an area 
over which the preceding deposits were but thinly accumulated. 
In the period of the Kaskaskia limestone, we find also a maximum 
development of animal life, and the grand culmination of that fauna 
which preceded the Coal measure period. 
The nature of these accumulations, the condition of the ocean bed, 
and the character of the fauna, with the varying limits of the waters 
of that period, are all so different from any conditions existing in the 
east during the interval between the Chemung group and of the Coal 
[ Paleontology III.] 8 
