46 The Badland Formations of the Black Hills Region 



in Washington County, Pine Ridge Indian reservation, South 

 Dakota. Concerning their occurrence here, Prof. Barbour, who 

 has visited the locality, says : "The mode of occurrence of these 

 crystals seems most unusual and remarkable. In a bed of sand 

 scarcely three feet thick, and so soft as to resemble the sand on 

 the seashore, occur these crystals in numbers which can best 

 be figured in tons. We dug them out with our bare hands. 

 They are mostly single crystals, with numerous doublets, triplets, 

 quadruplets, and multiplets. In other words every form from 

 solitary crystals to crowded bunches and perfect . radiating con- 

 cretions were obtained. It was a matter of special interest in 

 the field to note that at the bottom of the layer the bulk of 

 these sand-lime crystals are solitary; one foot higher there is an 

 evident doubling of the crystals, until within another foot they 

 are in loosely crowded clusters, a little higher in closely crowded 

 continuous clusters, pried out in blocks with difficulty; still high- 

 er they occur in closely crowded concretions in contact with 

 one another, making nearly a solid rock. A little higher this 

 mineralizing process culminates in pipes, compound pipes, and 

 solid rocks, composed wholly of crystals, but so solidified that 

 their identity is lost, and is detected only by a certain reflection 

 of light, which differentiates the otherwise invisible units by 

 showing glistening hexagonal sections. There could not have 

 been a more gradual and beautiful transition, and all confined 

 to a bed six or eight feet in thickness."* 



The relation of the sand-calcite crystals to the Arikaree 

 concretions, as indicted above, discloses an important step in 

 the development of concretions in general, and doubtless to some 

 such cause as this crystallographic tendency is due the develop- 

 ment of the concretions of other strata, such as the Protoceras 

 beds and the Oreodon beds. 



SANDSTONE DIKES AND CHALCEDONY VEINS 



Dikes and veins are ordinarily elongate, vertical, or nearly 

 vertical rock or mineral masses occupying fissures in a pre-exist- 

 ing rock. The filling body, if intruded as an igneous rock while 

 in the molten condition, is commonly referred to as a dike. If 

 filled in by slow process of deposition from aqueous solution it 

 is known as a vein. It is now recognized that fissures sometimes 

 become filled with clastic material derived from adjacent or near- 



*Barbour, E. H. Sand Crystals and their Relations to Certain 

 Concretionary Forms. Bull. Geol. Soc. Am., Vol. 12, 1901, p>p. 13-18. 



