-t"> ( > Van Tuijl — The Origin of Chert. 



variation. This is especially true of the chert in a bed of 

 crinoidal limestone, 18 feet in thickness, at the very top 

 of the formation. 



Significance of beds of silicified oolites.-. — It has long 

 been known that under favorable conditions beds of cal- 

 careous oolite may be changed over to siliceous oolite by 

 replacement. The writer has several specimens of 

 oolite in his collection which furnish irrefutable evidence 

 of silicification. Nearly every stage in the process is 

 illustrated. Attempts have been made to demonstrate 

 that some siliceous oolites represent primary deposits. 

 While this may be true in certain cases, it certainly can- 

 not hold for all siliceous oolites. If, then, it is demon- 

 strated that oolitic limestone may be silicified under 

 favorable conditions, is it not possible that ordinary lime- 

 stone may undergo the same transformation? 



Time and Place of Replacement. 



As regards the conditions which favor replacement of 

 limestone it is believed that in the case of most secondary 

 cherts the silicification has proceeded on the bed of the 

 sea, possibly contemporaneously with the deposition of 

 the limestone. . If this is true the occurrence of chert 

 nodules within layers of compact limestone is not difficult 

 to explain upon the basis of the replacement theory. An 

 exception to the belief that most cherts are formed very 

 early is furnished by the above described occurrences of 

 chert along fissures. These have clearly been formed 

 subsequent to the recrystallization of the limestone. 



The problem of the source of the silica which has been 

 segregated to form secondary cherts will probably 

 always be in dispute. A number of years ago the writer 

 gave this subject some attention while making a petro- 

 graphic study of the Mississippian cherts of southeastern 

 Iowa. In a brief paper which was published at that 

 time 7 the paucity of the remains of siliceous organisms 

 in the limestone was remarked upon, and the following 

 conclusion was reached : 



"It is probable, therefore, that much, if not all, of the 

 silica is of inorganic origin, having been deposited in a 

 collodial condition upon the bed of the sea while the 

 limestone w T as being formed." 



Colorado School of Mines. 



T Proe. Iowa Aca.l. Sci., vol. 19, pp. 173-174, 1912. 



