380 W. A. TARR SYNGENETIC ORIGIN OF CONCRETIONS IN SHALE 



of being uniformly distributed must be determined by future studies. It 

 is not improbable that an excessively rapid precipitation would mean a 

 bed of limestone and a slower rate would result in the material assuming 

 a concretionary form. It is apparent that the successive recurrence of 

 planes of calcareous concretions and the recurrence of successive beds of 

 limestone have so much in common that the above view as to their syn- 

 genetic origin of the concretions in shales is just as probable as the view 

 of the syngenetic origin of beds of limestone when both occur interbedded 

 with shales. 



In all the instances where the relationship of the concretion to the 

 surrounding beds could be studied it was found that the lines of stratifi- 

 cation went around the concretion. When the major part of these lines 

 go over the concretion it suggests a syngenetic origin with subsequent 

 burial. Growth subsequent to deposition would produce an arching of 

 the beds, unless the concretion is the result of cementation. If the con- 

 cretion is spherical, as the majority of them are, this arching of the beds 

 would leave a y-shaped opening entirely surrounding the form. This 

 opening should be filled if laterally moving solutions were bringing in 

 material and the concretion would lose its spherical form. This type is 

 rare. In the concretions from the Pennsylvanian in Missouri this space 

 was filled by thin lenses of shale, the material having accumulated around 

 them after they had attained their present size (see figure 1). 



When the lines of stratification curve around the concretion and when 

 the concretion shows lines of stratification which are independent of 

 those around it, the evidence appears convincingly favorable to a synge- 

 netic origin. The stratification lines in the concretion developed as the 

 material which formed it (largely silica and kaolin in the Mowry shale 

 member in the northern Black Hills) accumulated. With a fresh influx 

 of mud, further growth was cut off and the concretion was buried. 



The slickensides on some concretions are open to more than one inter- 

 pretation. If considered alone, they could be due to the beds being 

 pushed up by growth or to the beds slipping down around the concretion. 

 When studied in connection with the arching of the beds and the lack of 

 lines of stratification passing through the concretion, they are taken to 

 mean that the beds during consolidation settled down around the concre- 

 tion. Further, when the concretions are of the same material as the 

 shale, largely kaolin and silica, and are not due to cementation, their 

 volume (see below) favors a syngenetic origin, as it is impossible that 

 they could have displaced so much rock by pushing it aside. 



