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



rather thin near the edges. The concretions are composed of shale and 

 silica and are very hard. They lie along one bedding plane, but there is 

 no regularity in the lateral spacing or in their size. Occasionally they 

 show a very fine septarian development. A band of cone-in-cone fre- 

 quently occurs along the top of these concretions. This band in some 

 instances was three inches thick. 



The bedding planes in those studied curve under and over the concre- 

 tions. The concretions themselves show parallel lines of stratification, 

 hut they end with the concretion and are not continuous or connected 

 with those of the surrounding beds (see figure 2). There is apparently 

 but one conclusion to draw from such occurrence, and that is that the 

 concretion antedates the overlying beds and is subsequent to the beds it 

 rests on, since its own banding is independent of both. 



CONCRETIONS IN THE CRETACEOUS (CARLISLE) SHALES IN SOUTH DAKOTA 



AND WYOMING 



Concretions are very abundant in the Carlisle shale in the northern 

 Black Hills region. The formation is a dark gray to black shale about 

 600 feet thick. The concretions are especially well developed in the lower 

 and upper parts. Near the base are several lines of flat, elliptical con- 

 cretions, around which the bedding planes pass. A short distance above 

 these are some very large, more or less spherical concretions, which attain 

 a diameter of several feet. They are composed of sandy limestone and 

 are horizontally banded like the adjacent shales. Septarian cracks give 

 a radial and concentric appearance to them. Horizontal bands of cone- 

 in-cone, 2y2 inches wide, run entirely across some of them. None of 

 these concretions were seen entirely surrounded by the shale. The most 

 notable point is their distribution along a definite horizon. 



The upper portion of the Carlisle shale is full of concretions, which 

 occur consistently along one bedding plane, the successive lines of con- 

 cretions occurring at intervals of from 15 to 30 feet and averaging about 

 20 feet apart. These concretions range up to 18 inches in thickness and 

 may be 10 feet long. This length is due mainly to adjacent concretions 

 coalescing. They are composed of a fine-grained gray limestone or calcite 

 and all show septarian cracks which are filled with a yellow to deep brown 

 calcite. 



There is an immense numlier of these concretions in this portion of the 

 sliale, and although they are always along definite planes they are irregu- 

 larly spaced on these planes. The significance of their occurrence and 

 distribution will be noted under oriafin. 



