Van Tuyl — The Origin of Chert. 451 



Other irregular masses of chert have been observed 

 in the Mississippian limestones of Iowa, especially in the 

 Montrose chert, which has formerly been referred to the 

 Upper Burlington, but which is clearly basal Keokuk, as 

 indicated by its fauna. This has been carefully studied 

 by the writer in the excavation below the bed of the 

 Mississippi River for the dam at Keokuk, Iowa. The 

 accompanying section will give the reader an idea of 

 the character of the chert at this place. Conditions were 

 exceptionally favorable to the study of the chert here, 

 because weathering had not obscured the original rela- 

 tionships of this material and the limestone. 



Section of Montrose Chert in Excavation below bed of Mississippi River 

 at Keokuk, Iowa. 



Thickness Approximate per- 

 Ft. In. centage of chert 



13. Limestone, dark gray, impreg- 

 nated with small, irregular patches of 

 chert and chalcedony, and containing 

 small, imperfect calcareous geodes .... 8 50 



12. Limestone, ash-colored, impure, 

 very fine-grained. Chert correspond- 

 ingly fine-grained, and occurring as dis- 

 continuous seams which usually run 

 parallel to the bedding 3 8 30 



11. Chert, white, gray, and bluish, 

 dense, with discontinuous bands and 

 irregular pockets of dark gray crinoidal 

 limestone. Occasional geodic cavities in 

 the chert are lined with drusy quartz 

 studded with rhombs of calcite 1 3 75 



10. Limestone, dark gray, crinoidal, 

 fine-grained, almost entirely replaced 

 by dark gray chert locally 1 9 50-75 



9. Limestone, impure, very fine- 

 grained, ash colored, with pockets and 

 patches of bluish to whitish chert .... 5 33 



8. Limestone, dark gray, crinoidal 

 cherty 9 33 



7. Limestone, dark gray, bearing 

 small crinoid stems, with whitish chert 

 band in middle 2 50 



6. Limestone, very fine-grained, ash 

 colored, bearing irregular patches of 

 whitish chert 1 2 25 



