CBETACEOTTS SYSTEM. 293 



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vertebrate remains. Inoceramus problematicus is occasion- 

 ally met with, and also a few other imperfect specimens of 

 other LamellidbrancMates. Of remains of vegetation, leaves 

 of Salix MeeJcii and Sassafras Cretaceum have been occa- 

 sionally found. 



4. THE INOCERAMUS BEDS. 



These beds have in all a maximum thickness in Iowa of 

 about fifty feet, and rest directly upon the Woodbury sand- 

 stones and shales. They have not yet been observed in 

 Iowa, anywhere, except in the bluffs which border the Big 

 Sioux river in Woodbury and Plymouth counties. Although 

 not exposed to view, they doubtless also exist in the bluffs 

 that skirt the Big Sioux river all the way to the northwest 

 corner of the State. 



They are composed almost entirely of calcareous material, 

 but it is not true, compact limestone. This division is both 

 lithologically and palseontologically more truly character- 

 istic of the Cretaceous age than either of the preceding 

 divisions. 



These beds, although their aggregate thickness is so little, 

 may be properly subdivided into three portions. The first 

 and lowest consists of fine, soft, indistinctly stratified, calca- 

 reous material somewhat resembling clay, but which really 

 contains very little true clay. Second, regularly and thinly 

 bedded calcareous material, much of it having all the cha- 

 racters of true chalk, except that the greater part of it is 

 much colored with oxyd of iron which renders it unfit for 

 practical use. The third and upper portion may be called a 

 shaly and chalky limestone ; too hard to be called chalk 

 and too chalky to be called limestone. It is, however, as is 

 also nearly all of the material of the Inoceramus beds, nearly 

 pure carbonate of lime. 



Economic Value. The material of the upper portion just 

 referred to is used extensively for the production of lime, the 

 quality of which is equal to that obtained from common 

 limestone. Xo building material at all is to be obtained 



