56 THE WHITE RIVER BADLANDS 



CONCRETIONS, SAND CRYSTALS, DIKES, VEINS 

 AND GEODES 



Concretions. A concretion is a spherical, cylindrical, 

 elliptical, or nodular body produced by the tendency of cer- 

 tain mineral constituents to orderly aggregate about a 

 common center within an embedding rock mass. The dis- 

 covery in the White River badlands several years ago of 

 what are known as sand or sand-calcite crystals has added 

 much to our knowledge of concretionary development and 

 has served well to indicate the local conditions with refer- 

 ence to these abundant and interesting forms. 



Concretions vary greatly in size, shape, composition, 

 manner of distribution and method of growth. They are 

 common in the Great plains formations. In some of the 

 Cretaceous and Tertiary beds they may be found in prodi- 

 gious numbers. They occur in many places and in various 

 horizons and of all sizes up to several feet in diameter. Any 

 horizon which contains the concretions at all is likely to 

 contain many of them and often they coalesce horizontally 

 and form continuous strata. More frequently they are 

 separate and, being harder than the surrounding material, 

 they often tend under the influence of erosion to become 

 the caps of earth pillars. The material of which they are 

 made is generally an arenaceous clay with calcium car- 

 bonate as a cementing material, but iron oxide is often 

 times present in considerable quantity. 



Sand Crystals. The sand crystals are made up of ap- 

 proximately sixty per cent of sand and about forty per 

 cent of calcium carbonate. The former occurs as an in- 

 clusion, while the latter, the mineralizing agent, is the 

 crystal proper. The size varies in length from a quarter of 

 an inch or less to fifteen inches. They occur chiefly in the 

 Arikaree formation, which is largely a soft sandstone. 

 Much of the rock is concretionary, and not a little of it is in 

 cylindrical or pipe-like masses, often many feet or yards in 

 length. These often disclose evidence of some internal 

 molecular or crystalline arrangement and weathered speci- 

 mens not infrequently show a radiate or rosetted structure, 

 due to the tendency of lime-salts to crystallize according to 

 the laws governing calcite as far as the interference in the 

 part of the sand grains will allow. ( Plate 52 ) . 



