CARBONIFEROUS SYSTEM. 241 



if it did its remains seem not to have been deposited in Iowa 

 strata of later date. Specimens of it are rare in the last 

 named formation, and have not yet been found at all in the 

 Up er coal-measures. 



Among the shales of this formation one not unfrequently 

 finds what are evidently markings or remains of seaweeds, 

 but they are rsually too indistinct for satisfactory classifica- 

 tion. Some interesting specimens of the impressions of 

 fucoids, closely resembling the Fucoides canda-galli of the 

 Devonian rocks of New York, have been found in the calca- 

 reous sandy shales the Lower coal-measures in Wapello 

 county. They are probably of the same species as those 

 described by Prof. Leo Lesquereux, from the coal-measures 

 of Lawrence county, Pennsylvania, under the name of Cauler- 

 pites marginatis. Markings of Lepidodendron and Catamites 

 have been found in the coal itself of this formation, as well 

 as an abundance of the remains of woody fibre, that closely 

 resembles pine wood charcoal. 



2. MIDDLE COAL-MEASURES. 



The passage of the strata of the Lower, into the Middle coal- 

 measures is not marked by any well defined line of division, 

 but considering the strata in the aggregate, they each possess 

 characters sufficiently distinct to warrant their separation. 



The area occupied by this formation is smaller than that of 

 either the Lower or Upper, and constitutes a narrow region 

 between them. It has been carefully studied by Prof. St. 

 John, whose report upon it will be found in the next 

 chapter. , 



3. UPPER COAL-MEASURES. 



Area and General characters. The area occupied by the 

 Upper coal-nleasure formation in Iowa is very great, com- 

 prising thirteen whole counties in the southwestern part of 

 the State, together with parts of seven or eight others adjoin- 

 ing. It adjoins bv its northern and eastern boundary the 

 PI 



