454 Van Tuyl — The Origin of Chert. 



material were seen in some of the rocky knobs protruding 

 through the drift near the northeast corner of section 27, 

 Bremen township. There seems to be here a solid bed of 

 chert, and great detached blocks, eighteen inches thick, 

 and three or four feet in length and width, lie heaped on 

 each other or scattered over the surrounding surface. 

 The large blocks referred to showed no definite traces of 

 fossils, but thousands of tons of silicified corals are 

 embedded in the very thin drift and residual clay cover- 

 ing the adjacent fields. Vast numbers of these have been 

 gathered and piled along the roadway into a rude stone 

 wall, ten or twelve feet wide at base and several rods 

 in length." 



The writer has observed a similar relationship in an 

 outcrop of Niagaran dolomitic limestone, ten feet in 

 height, along a small creek, about one-half mile north- 

 east of Hazleton, in Buchanan County, Iowa. Much 

 chert is present in the form of nodules which are most 

 abundant along the bedding planes, and associated with 

 it are many fossils, especially corals. Most of these are 

 silicified, but some of them are still calcareous. 



Very similar relations were found in Niagaran lime- 

 stone at the Flint Rapids of the Ekwan River, in the 

 Patricia district of northern Ontario. At this locality, a 

 bed of coralline limestone contains dark chert nodules, 

 and many of the corals, even specimens situated several 

 feet from the nodules, have been silicified. 



This association is illustrated to a lesser degree in the 

 Lower St. Louis limestone of southeastern Iowa. In a 

 thin zone near the top of this limestone, the coral Litho- 

 strotion canadense is frequently associated with chert 

 nodules, and is then almost invariably silicified. 



Replaced fossils in chert. — Many of the cherts which 

 have come to the writer's attention contain silicified fos- 

 sils, but the structure of these is frequently so obliterated 

 that it is difficult to recognize them. It has been the 

 writer's experience that many of the compact and dense 

 cherts which appear to possess no definite structure will 

 show traces of silicified fossils upon close inspection. 

 These are much more distinct after the chert has been 

 subjected to weathering. At times these included fossils 

 still consist of calcite, or are only partially replaced 

 by silica. In such cases, the calcareous material is soon 

 removed by solution upon exposure, leaving only molds in 



