CARBONIFEROUS SYSTEM. 207 



fishes related to the sharks of the present seas. The others, 

 which are rare, consist mainly of the dermal plates of Ganoids 

 — fishes somewhat related to the gar-pike of the Mississippi 

 and its tributaries. No remains of bony fishes, like those 

 most common at the present day, are found in these rocks, 

 and it is believed that none such existed at the time the 

 material of those rocks were being deposited. 



Besides the occurrence of fish remains as isolated specimens 

 at various localities, and throughout the whole vertical range 

 of the formation, they are particularly numerous at some 

 limited localities, where they are sometimes found crowding 

 the entire substance of a single stratum for a considerable (Us- 

 ance. It has been supposed that the strata of this formation, 

 thus charged with these remains as seen at many and distant 

 localities, were parts of one continuous and uninterrupted 

 horizon, all having had a simultaneous origin. It was 

 thought that this assumed horizon was to be recognized at 

 many localities in southeastern Iowa and western Illinois, 

 some of which are more than a hundred miles apart. It is 

 more probable, however, that there are many horizons in 

 the formation, characterized by a profusion of fish remains, 

 all of which are comparatively quite limited in extent. 



On Burlington creek near Columbus City, in Louisa county, 

 there is a stratum in an exposure of this limestone having a 

 thickness of from eight to twelve inches, so fully charged with 

 these remains, that a part of it might with propriety be called 

 bone breccia.* Above and below the stratum are the ordinary 

 firm layers of Burlington limestone. The fish-stratum itself 

 is nearly of the same texture, but a part of it is soft and 

 friable, and sometimes slightly magnesian. The rock, as 

 usual, is colored yellowish with per-oxyd of iron, and where 

 the fish remains are the most abundant, it has a greenish 

 tinge, with numerous specks of deeper green. A qualitative 

 analysis of this greenish rock by Prof. Emery, shows the 



* This interesting locality was discovered by Frank Springer, Esq., of Burlington, 

 who has made for himself a fine collection of all species found there. 



