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



Other citations are possible/*' but these are sufficient to indicate the 

 line of thought as expressed in recent text-books and articles. 



There is abundant evidence that concretions in sandstones are epige- 

 netic, and very probably the same is true of concretions in any permeable 

 formation. Shale, in contrast to sandstone, is an impermeable rock, as 

 is proven by its being the confining formation for underground waters 

 and for oil and gas. Concretions in shale, therefore, present a more diffi- 

 cult problem. As indicated by the above quotations, where the bedding 

 planes of the inclosing rock pass directly through the concretion, cementa- 

 tion of the rock material in place is evident. The discoid form so com- 

 mon in concretions has been interpreted to confirm the same method of 

 origin, deposition following the bedding planes. This form, however, 

 may be due to flattening by pressure, as the writer has shown to be the 

 case in chert concretions in the Burlington limestone.^^ Strong evidence 

 of an epigenetic origin would be the sharp crumpling of the beds at the 

 sides of the concretion as it expanded through lateral growth. Singu- 

 larly enough, such crumpling is absent from all concretions in shale which 

 the writer has been able to examine. 



Field Studies of Concretions in Shale 

 concretions in the pennsylvanian (cherokee) shales in missouri 



Concretions occur in a three-foot bed of black shale in the Pennsyl- 

 vanian shales in Boone County, Missouri. The concretions range in 

 diameter from one-quarter of an inch to twelve inches. They are com- 

 posed of kaolin and silica and are only slightly calcareous. The larger 

 concretions contain well preserved fossils in the outer part or on the 

 upper surface. Usually the concretions are elliptical in cross-section and 

 circular to elliptical in plan. Irregular forms also occur and many are 

 rudely symmetrical. 



The relationship of the concretions to their surrounding beds is of 

 especial interest. The concretion rests in a shallow cup and the remain- 

 ing beds arch over it. No stratification lines pass through the concre- 

 tions. Since the greater thickness of beds arches over the concretion, the 

 bend at the sides would be very abrupt, were it not for the fact that small 

 lenticular beds lie in this pocket next to the concretion. These rapidly 

 pinch out away from the concretion and are apparently due to material 



1" The writer's attention has been called to an excellent article by J. M. Arms Sheldon, 

 "Concretions from Champlain clays of the Connecticut Valley," Boston, 1900, which has 

 a very fine bibliography, but he has been unable to see the original. 



^1 W. A. Tarr : Origin of the chert in the Burlington limestone. Am. .lour. Sci., vol. 

 xliv, 1917. p. 438. 



