166 E. H. BARBOUR — SAND CRYSTALS 



buttes which are so prominent in southern South Dakota, northern and 

 western Nebraska, and eastern W^^oming. 



Mode of Occurrence. 



The formation is characterized b}" an abundance of concretions gen- 

 erally having the form of horizontal cylindrical masses which project 

 from the walls of the buttes like guns from a fortress, and also by the 

 occurrence in its upper member of numerous rootlets and vegetal fibers- 

 There are concretions, as round as cannon balls, often scattered thickly 

 over many acres of ground. These, however, usually pass into great 

 aggregates of cylindrical form called pipes. , 



It is interesting to note the occurrence of ever}' condition and ever}'" 

 possible gradation from solitary spherical concretions to strings of partly 

 united concretions, then to pipes, which we shall consider as made up 

 of an infinite series of spherical concretions. Wind and rain supi)le- 



Fir.uRE 1. — Simple and compixind Conrrrtions and Pipes, northwestern Nebraska. 



ment each other in exposing considerable tracts of these concretions, 

 and, in the case of the compound ones, which are sufficiently lithified 

 to withstand long weathering, the exposure simulates the planed and 

 deeply grooved surface of a glaciated region. 



Physical Characters of the Concretions 



The individual pipes vary in size from a hand specimen to those ex- 

 ceeding a hundred yards or more in length, and from the simple to the 

 compound pipes. When weathered out, these impart to the landscape 

 a singularly ragged and fantastic appearance even surpassing the weird 

 effects produced by the erosion figures known as " hoodoos.'' 



An exposure of the ordinar}" spherical t3'pe, if scrutinized, reveals the 

 fact that man37^ or all of the concretions are obscurely or distinctly radiate 

 like radiate calcite. This is but the visible evidence of some internal 

 molecular or crystalline arrangement. Even the loose and less coherent 

 matrix reveals, under the action of wind and rain, an ill-defined, though 

 unmistakably radiate, or rosetted structure. This eff'ect is cr3'stall()- 

 graphic, and is due, as is shown by inspection, solely to the calcium car- 

 bonate cement. At some stage of development the sands were evidentl}^ 



