Van Tttyl — The Origin of Chert. 455 



the chert. Such molds are very abundant in the Montrose 

 chert of southeastern Iowa. 



Preservation of structure and textiles. — In excep- 

 tional cases chert shows structures and textures almost 

 identical with those of the associated limestones. This 

 is illustrated by some of the cherts of the Montrose chert 

 horizon, and the Keokuk limestone in Iowa. In these 

 formations the texture of crinoidal layers and even the 

 color is frequently retained by the chert. This is 

 especially true in cases where fragments of fossils have 

 not been wholly replaced. In more complete stages of 

 silicification, the structures and textures are usually lost. 



Ziegler 3 has described chert concretions in oolitic 

 limestones of Ordovician age, near Bellefonte, Penn. 

 These preserve the original oolitic structure and he inter- 

 prets them as having been formed by replacement. 



Chert deposits not occupying definite zones. — If we 

 regard chert deposits as having been formed by primary 

 deposition, we should expect these deposits to appear at 

 the same horizon over a considerable area. This appears 

 to be true in case of some cherty formations, but it does 

 not hold for all. For example, Calvin has shown 6 that 

 the cherts of the Niagaran dolomite of Delaware county, 

 Iowa, occupy no definite horizon. I quote from his 

 description : 



''Chert occurs extensively as concretions in the layers 

 of Niagara Limestone or as partings between them. Its 

 distribution, however, both horizontally and vertically, 

 is very erratic. In some localities, at certain horizons, it 

 is present in enormous quantities, making up fully half, 

 or much more than half, of the entire mass of rock 

 exposed ; in other localities at the same horizon it may be 

 entirely absent. ' ' 



The failure of chert to follow definite horizons in the 

 Keokuk limestone has been demonstrated in connection 

 with the detailed work on the stratigraphy of that forma- 

 tion in Iowa for the State Geological Survey. At some 

 localities certain beds in this formation are filled with 

 chert, but only a few miles away these same horizons 

 may show no trace of this rock, or perhaps only a few 

 scattered nodules. The amount of chert in certain hori- 

 zons of the Burlington limestone of Iowa shows a similar 



5 This Journal, vol. 34, p. 121 ff., 1912. 

 °Iowa Geol. Sur., vol. 8, p. 158. 



