104 IOWA ACADEMY OF SCIENCES. 



That the cherts may have been secondary features, 

 formed long after the rocks containing them were laid 

 down and consolidated, does not seem to have occurred to 

 any of the writers mentioned. That some of the Carbon- 

 iferous cherts of Iowa and Missouri have been formed in 

 the rocks long after the latter were formed there is small 

 doubt, as subsequent statements will conclusively show. 

 HoM^ extensive this secondary mode of chert formation is, 

 is not as yet definitely known. It is believed that the 

 process is of wide application, if not universal, so far at 

 least as concerns the Carboniferous cherts of the two states 

 mentioned. 



In southwestern Missouri, in the mineral belt, the cherts 

 constitute extensive layers interbedded with limestones. 

 In many cases the proportion of chert greatly exceeds that 

 of the limestone. So marked is this preponderance of 

 silicious beds that the formations or terranes are called 

 "cherts" in place of "limestones."' 



Were one to study the cherts of southwest Missouri 

 alone, he might long look for evidence for any other 

 explanation of genesis than that of a formation contem- 

 poraneous with the geological terrane. We get an import- 

 ant clue, arguing for non-contemporaneity of formation of 

 the chert and inclosing limestone, in a consideration of the 

 lower carboniferous terianes of southeastern Iowa and 

 northeastern Missouri, when the chert constitutes a com- 

 paratively small part of the whole terrane. 



Careful comparisons of the fossils from these cherts and 

 those of the surrounding limestones show that the forms 

 to a great extent are identical. Moreover, numerous shells 

 and crinoids are found partly imbedded in the chert and 

 partly in the limestone, and fossils likewise half chert, half 

 limestone, with a sharp line of separation, indicating 

 clearly that the silicious impregnation was acquired long 

 after the original deposition of the beds, and was not due 

 to a greater silicity of the waters in which the calcareous 

 deposits were made, as has been held by many prominent 

 writers. This is in accordance with observations made 

 elsewhere in the Burlington limestone. 



