202 GENEKAL GEOLOGY. 



characters, that it is difficult to say where the one ends 

 and the other begins ; but the separation of the other forma- 

 tions of the Snb-carboniferons group from each other is 

 of a different character, being more abrupt and distinctly 

 defined, and very similar in all cases. 



This separation consists in the interposition of a series of 

 silicious beds between each of the distinctive limestone 

 formations, which, although they form an uninterrupted 

 stratigraphical passage from one to the other, constitute the 

 palseontological boundary between them. This change in 

 the lithological character of the deposit, which, commencing 

 gradually, foretold the close of the epoch, seems to have had 

 the effect to check, and finally to arrest the progress of those 

 forms of life which previously existed in great profusion. 

 Then with the resumption of the calcareous deposits that 

 ushered in the succeeding epoch, similar, but not identical 

 forms were introduced, which flourished and progressed until 

 checked and arrested again by similar deposits of silicious 

 strata that continued to accumulate until the close of another 

 epoch. 



Thus the accumulation of the strata which compose all the 

 formations of the Sub -carboniferous group in southeastern 

 Iowa, from the Lower Burlington limestone to the St. Louis 

 limestone, inclusive, was evidently uninterrupted, while the 

 epochs that passed during the time in which the accumu- 

 lation took place, are marked by lithological changes in 

 those strata, and also by the specific changes in their fossil 

 contents.* 



The Burlington limestone formation consists of two distinct 

 calcareous divisions which are separated by a series of 

 silicious beds, such as have been referred to as separating 

 each limestone formation of the group. These beds consist 

 of light grayish or yellowish layers of silicious shale and 



*This is the case in the southern part of the State where the series is complete as far 

 as it is represented, but, as will be seen further on, there is some unconformability 

 among the different formations of the group farther to the northward, as well as the 

 unconformability of the coal-measures upon them. 



