BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 32, pp. 373-384 DECEMBER 1, 1921 



SYNGENETIC OEIGIN OF CONCRETIONS IN SHALE ^ 



BY W. A. TAER 



{Bead before the Society December 28, 1920) 



CONTENTS 



Page 

 Introduction 373 



Prevalent views as to origin 374 



Field studies of concretions in shale 376 



Concretions in the Pennsylvanian (Cherokee) shales in Missouri .376 



Concretions in the Cretaceous (Graneros) shales in South Dakota and 



Wyoming 377 



Concretions in the Cretaceous (Carlisle) shales in South Dakota and 



Wyoming 378 



Concretions in the Cretaceous (Pierre) shales in South Dakota and 



Wyoming 379 



Concretions in the Cretaceous (Claggett) shales in Hill County, Mon- 

 tana, and southern Saskatchevpan, Canada 379 



Origin of the concretions ». . . 379 



Correlative evidence of syngenetic origin 381 



Fossiliferous concretions 381 



Physical factors 382 



Summary : . 383 



Introduction 



Concretions may be divided into two classes as to origin: (1) syn- 

 genetic, or those which were deposited or formed at the same time as the' 

 inclosing beds, and (2) epigenetic, or those which liave been formed sub- 

 sequent to the deposition of the surrounding beds. The syngenetic con- 

 cretions would be older than the beds under them and younger (for the 

 most part, at least) than the beds over them, and the epigenetic concre- 

 tions would be younger than any of the surrounding beds, because they 

 are due to growth by the addition of material from tltc surrounding rocks. 



^ Manuscript received by the Secretary of the Society April 16, 1!»21. 



(373) 



