324 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



Sandstone produces the largest concretions, as, for example, the 

 loglike structures from South Dakota reaching a length of a hun- 

 dred feet, and on the Great Plains the many cannonball forms, up 

 to 12 feet in diameter, which lend a characteristic aspect to the 

 scenery and, indeed, give rise to local names such as the Cannon Ball 

 River. 



Concretions are responsible for the curious structures known as 

 " Septaria ", so named from the partitions (septa) traversing them. 

 A clay concretion in the f)rocess of its formation will shrink as the 

 mud dries and hardens, and fractures radiating from the center, 

 often very regularly arranged, will ensue (pi. 1, figs. 9, 10). While 

 the filling of such shrinkage cracks with mineral matter is un- 

 doubtedly the cause of the septaria, the exact process is not under- 

 stood. Probably a concretion with a colloidal or gelatinous clay 

 center changes to a crystalline solid with accompanying contraction 

 and cracking; or possibly the expulsion of water from the saturated 

 central area produces the same result. The open spaces caused by 

 these fractures are later usually filled with some mineral matter 

 other than that forming the nodule (pi. 2, fig. 1). Should this be 

 more insoluble than the material of the original concretion, it will 

 stand out as ridged polygons when the nodule is subjected to 

 weathering. Or, should the more soluble concretion be entirely dis- 

 solved away, a curious framework of the mineral-filled fractures 

 remains (pi. 2, fig. 2). Septaria are particularly interesting to 

 the mineral collector, for the veins filling the fractures may yield 

 crystals of a variety of minerals, ranging from the metallic sul- 

 phides to the nonmetallics such as barite and selenite. Certain very 

 abundant small bodies termed pisolites and oolites have a concre- 

 tionary structure, but their origin is somewhat different. 



A third type of concretion closely resembling septaria externally 

 presents a most artistic appearance because the surface is ornamented 

 with large and small polygons arranged in a geometric design. Pos- 

 sibly such concretions (pi. 3, figs. 1, 2) were, like septaria, formed 

 by filling of shrinkage cracks, but they now show no evidence of such 

 an origin, for the polygonal marking is apparent only at the sur- 

 face. When the surface is smoothed by abrasion, only the solid 

 dense mineral is visible. The excellent example of this type here 

 figured came from some Cretaceous formation along the Cannon Ball 

 River of North Dakota, and was presented to the Smithsonian Insti- 

 tution by Percy Train. The exterior of this concretion is of limonite, 

 iron hydroxide, but the interior, according to Mr. Train, consists of 

 hematite radiating from a rounded water-worn boulder that served as 

 a nucleus, which in one instance was one-third the size of the speci- 

 men. Inspection of plate 3 will show that the upper surface of the 



