1 .">■_' Yioi Tv/yl — Tin Origin of Chert. 



Thickness Approximate per- 

 l''l. In. centage of chert 



5. Chert, in the form of a layer 

 about 7 inches thick which locally 

 grades into gray crinoidal limestone.. 7 25 



4. Limestone, fine-grained, with 

 scams and nodules of bluish and whitish 

 chert 1 5 25 



3. Limestone, gray, bearing whitish 

 fossiliferous chert as large, irregular , 



patches, and irregular discontinuous 

 bands 3 50 



2. Limestone, fine-grained, with nod- 

 ules of white and dark colored chert. . 1 30 



1. Limestone, dark gray, crinoidal, 

 becoming lighter colored, and purer 

 downward ; in layers 6 to 22 inches 

 thick, bearing fossiliferous chert in the 

 form of bands, lenses and nodules .... 8 30 



The chert of this horizon also outcrops about one- 

 fourth mile northeast of Augusta, Iowa. At this place 

 most of the chert masses are lenticular in shape, but 

 many are very irregular. Furthermore, some of the 

 bands and lenses of chert branch and reunite, and occa- 

 sionally two lenses are joined by vertical columns. 



Somewhat similar relations are shown by the Montrose 

 chert in an exposure in the bank of Lost Creek, about 

 two and one-half miles southeast of Denmark, Iowa, and 

 again in exposures along the small creeks two miles 

 northwest of the same town. . 



Patches of limestone in chert. — The presence of small 

 patches of limestone in chert masses furnishes strong evi- 

 dence of the replacement origin of all cherts showing 

 them. Many of these are irregular in outline, and their 

 relationship clearly favors the view 7 that they represent 

 residual masses of the limestone which for some unknown 

 reason were not replaced at the time of the surrounding 

 material was silicifiecl. Such limestone patches are 

 most abundant in the larger masses and bands of chert, 

 and have been found in nearly all important chert 

 deposits which have been examined by the writer. They 

 have been observed in the Montrose chert of south- 

 eastern Iowa near Augusta; at Keokuk; two miles 

 northwest of Denmark; and at nearly all other localities 

 where this formation has been studied. Such inclusions 

 occur also in the chert bods of the Galena dolomite on the 



