CAKB0NIFEK0US SYSTEM. 195 



counties, and along the Des Moines river in Humboldt 

 county. 



Although the southern part of the formation in Iowa has 

 the best development of all its distinguishing characteristics, 

 yet the width of the area it occupies is much greater in its 

 northern part, reaching a maximum width there of about 

 eighty miles. This increased width of area occupied by the 

 formation does not necessarily indicate an increased thickness 

 of it, but it is evidently caused by a broad fold of the strata in 

 that part of the State, which places them for that distance 

 almost exactly parallel with the general surface of the country, 

 and consequently without any perceptible dip to the westward. 



At Bunker's mill, on English river, near Richmond county, 

 the following section was measured, commencing at the water 

 level; 



No. 2, roughly bedded, earthy, yellowish limestone, with much silicious 

 cherty material, 12 feet. 



No. 1, bluish and bluish-green, indurated, sandy clays, 47 feet. 



No. 1 of this section is, without doubt, equivalent with No. 1 of the section at 

 Burlington. No. 2 of this section is the equivalent of No. 7 there, all the inter- 

 vening beds observed at Burlington are wanting at this locality. The charac- 

 teristic fossils of the formation were obtained from No. 1, but the material of No. 

 2 is not well suited to the preservation of fossils, if it ever contained any. 



Going northward from Washington county, no other 

 exposures of any of the sub-carboniferous formations are 

 found until the valley of the Iowa river is reached. Here the 

 river cuts obliquely across the whole area occupied by the 

 group, but no rocks except those referred to the Kinderhook 

 formation appear. It is possible that the true Burlington 

 limestone may extend so far northward, but no proof of it 

 has yet been observed. 



At Indiantown, in Tama county, the section, represented by 

 Figure 7, was measured, commencing at the water-level of the 

 Iowa river: 



