f 



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



This classification differs from that of Todd,- hut includes his four classes 

 in two, gTouping them on a hasis of their age relative to the surrounding 

 rocks rather than on the movement of the materials in the concretions 

 proper. Grabau-'' follows Todd in his classification. 



There is evidence to support each class, but in the main the published 

 views favor an epigenetic origin. Within the last two years the writer 

 has encountered evidence which appears to strongly favor a syngenetic 

 origin for concretions in shales. This evidence will be presented below 

 after a brief statement of prevalent views. It should be noted that this 

 paper deals mainly with the syngenetic origin of concretions rather than 

 with the origin of the concretionary form itself. 



Prevalent Views as to Origin 



The recent literature bearing on the origin of concretions, though not 

 abundant, indicates that the prevalent view favors an epigenetic origin. 

 Pirsson and Schuchert^ state : 



"While some are very pure, they [the concretions] often contain large amounts 

 of rock material, and in some cases the planes of stratification can be seen 

 passing through them. Their origin appears to be due to material in the rock 

 having gone into solution, and then for some reason having been steadily re- 

 deposited around certain centers as neuclei, thus building up the concretions." 



Chamberlin and Salisbury^ make the tAvo following statements : 



"The concretion may be made up almost wholly of concentrated matter, in 

 which case the matter originally in the place of the concretion has been 

 crowded aside ; or it may involve much of the material of the embedding rock. 

 . . . Concretions of the sort indicated above often develop after the inclos- 

 ing sedimentary rock was deposited. This is shown, among other things, by 

 the fact that numerous planes of lamination may sometimes be traced through 

 the concretions. Concretions also form in water during the deposition of sedi- 

 mentary I'ock." 



They refer in the last sentence to oolites and pisolites. 

 Cleland*^ states that concretions may be of both syngenetic and epi- 

 genetic origin. He states : 



"They are often formed before the rock containing them is hardened [indu- 

 rated], as is shown by the fact that (1) they are often cut by joints, and (2) 



"J. E. Todd: Concretions and their geologicil effects. Bull. Geol. Soc. Am., vol. 14, 

 1903, pp. 353-368. 



3 A. W. Grabau : Principles of stratigraphy, 1913, pp. 718 and 763. 

 * L. V. Pirsson and Charles Schuchert : Text-boolJ of geology. 1915. p. 273. 

 ^ T. C. Chamberlin and R. D. Salisbury : Geology, vol. i, pp. 492 and 496. 

 " H. F. Cleland : Geology, physical and historical, 1916, p. 77. 



