INTRODUCTION. 8f 



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 th^ 

 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 formoition 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 



[ Pal^ontoloot III.] 8 



