South . Dakota School of Mines 45 



tiary beds of the plains country they may be found in prodigious 

 numbers. They occur in many places and in various horizons 

 in the badland formations 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 horizon- 

 tally and form continuous strata. More frequently they are sep- 

 arate 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 carbonate as a cementing ma- 

 terial, but iron oxide is often times present in considerable 

 quantity. 



The sand-calcite crystals were first studied by Prof. E. H. 

 Barbour, of the University of Nebraska, in 1893. Professors 

 S. L. Penfield and W. E. Ford of Yale University, in 1900, de- 

 scribed a few additional forms. Later in the same year, Prof. 

 Barbour described the crystals more fully and gave also a brief 

 description of the locality, the geology, and the mode of occur- 

 rence, and showed the relation of the crystals to ordinary con- 

 cretions.* 



The crystals are made up of approximately sixty per cent 

 of sand and about forty per cent of calcium carbonate. The 

 former occurs as an inclusion, 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. Plate 15 shows sev- 

 eral characteristic forms. 



The crystals occur chiefly in the Arikaree formation, which 

 is largely a soft sandstone. Much of the rock is in concretion- 

 ary form, and not a little of it is in cylindrical or pipe-like 

 masses, often many feet or yards in length. These, according 

 to Barbour, often disclose evidence of some internal molecular 

 or crystalline arrangement and weathered specimens not infre- 

 quently show a radiate or rosetted structure, due to the tendency 

 of lime-salts to crystallize according to the laws governing cal- 

 cite as far as the interference on the part of the sand grains will 

 allow. 



The first discovered and most noted locality is at Devils 

 Hill, near Corn creek, about twentv miles south of White river 



*For a etill later discussion, including description of slightly 

 different shaped crystals and a record of the distribution and geologic 

 range of deposits of this character, the reader is refered to the fol- 

 lowing: Barbour, E. H., and Fisher, C. A. A New Form of Calcite- 

 Sand Crystal. Am. Jour. Sci., Vol. 14, 1902, pp. 451-454. 



