CORRELATIVE EVIDENCE 383 



The task of aggregating through ground-water action the enormous 

 volume of calcareous material involved in the limestone concretions in 

 the Carlisle shale, for instance, where they are very numerous along 

 planes at an average interval of 20 feet, would have heen a Herculean 

 task, even for ground-water. Had the calcareous material ])een distrih- 

 uted originally along one pla]ie, then tlie concretions should be elliptical 

 in cross-section, as the major part of the deposition would be at the sides. 

 If the calcareous material was originally distributed throughout the 

 shale, the concretions should be irregularly distributed, for the uniformity 

 of the shale would not favor segregation b}^ ground-water along certain 

 planes. This distribution along one plane and of successive planes of 

 concretions, if one accej^ts a syngenetic origin of them, would be depend- 

 ent on the laws controlling an alternating series of shales and limestones, 

 or that controlling the deposition of colloidal silica, as chert. 



The volume of some of the large concretions in shales should be con- 

 sidered. The materials composing the shale may be regarded as insol- 

 uble, as far as this problem is concerned. A volume of shale equal to that 

 of the concretions must be displaced. Taking a large concretion, such as 

 those in the loAver part of the Carlisle, where some concretions are 5 to 6 

 feet in diameter, the volume ranges from 65 to 315 cubic feet. Com- 

 pression due to expansion by growth can hardly account for the develop- 

 ment of this space, and neither would many of us accept the view that 

 this volume of shale had been removed in solution. A syngenetic origin 

 does not encounter this objection. 



Summary 



Most concretions which occur in shales are probably of syngenetic ori- 

 gin. Concretions found in the Pennsylvanian in Missouri and Hlinois 

 and in the Cretaceous in South Dakota, Wyoming, Montana, and south- 

 ern Saskatchewan present evidence which strongly supports this view. 



This evidence is as follows : 



(1) The concretions are arranged along one plane. 



(2) There are frequently successive planes of concretions in a forma- 

 tion. 



(3) Their volume is too large to l^e accounted for ]jy an epigenetic 

 growth, witliout cementation of previously existing materials. 



(4) The inclosing beds arch over the concretions. 



(5) The sides are slickensided by a slipping down of the beds around 

 the concretion. 



(6) In a few instances it Avas determined that the stratification planes 

 in the concretions were distinct from those in the inclosing: beds. 



