THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. X-XXN .— Origin of the Chert in the Burlington Lime- 

 stone ; hy W. A. Tare. 



Table of Contents. 

 Introduction. 

 I. Theories as to Origin. 

 II. The Chert in the Burlington Formation. 



A. Definition of chert. 



B. Megascopic description. 



C. Microscopic description. 



D. Mode of occurrence of the chert. 



E. Age of the chert. 

 III. Origin of the Chert. 



A. Outline of theory. 



B. Evidence for theory. 



1. Source of the silica. 



(a) Carried to the sea by streams 



(6) Derived from the land through chemical denudation. 



(c) Eelationship to periods of peneplanation. 



{d) Derived from shore work. 



2. Dispersion of silica in sea-water. 



3. Deposition of the silica. 



(a) Experimental. 



(6) Cause. 



(c) Form of precipitated silica. 



|d) Associated minerals. 



4. Relationship of chert to enclosing rock. 



5. Fossils and the silica, 



6. Absence of siliceous organisms in the Burlington formation. 



7. Conclusions as to origin under the theory. 



8. Evidence against the replacement theory. 



(a) Position of the chert in the limestone. 

 (6) No adequate source of the silica, 

 (c) Adv^erse evidence of the structure of the nodules. 

 {d) Fossils in the chert. 

 (e) Weathering of the chert. 

 (/) Conclusions as to replacement, 

 IV. Application of the Colloidal Precipitatipn Theory to Other Cherty 



Formations. 

 V. Summary and Conclusions. 



Am. Joue. Sci.— Fourth Series, Yol. XLIV, No. 264.— December, 1917. 

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