SOUTH DAKOTA SCHOOL OF MINES 59 



part way, leaving a considerable cavity and then the geode 

 when broken presents a crystal lining of mnch beauty and 

 interest. Commonly the geodes are more or less siliceous, 

 especially in the outer portions and, resisting weathering 

 better than the enclosing rock mass, may often be found 

 freed from the matrix lying on the disintegrating surface. 

 Not infrequently crystal fragments become detached within 

 the shell, and these, striking against the inner walls when 

 the geode is shaken, serve to make a sound. For this reason 

 the geodes are often referred to locally as rattle stones. 



Many geodes have been collected from the Big Bad- 

 lands. The diameter varies from one inch or less to several 

 inches. The prettiest ones of rather small size are found 

 near Imlay. They have commonly an irregular shell of 

 chalcedony more or less filled with bright clear-cut white 

 or colorless quartz crystals, the latter varying from micro- 

 scopic size to one-half inch or more in length. The finer 

 white crystals much resemble white sugar, hence the name 

 sugar geodes. Selenite (crystalized gypsum) is occasion- 

 ally present. The origin of the geodes is doubtless closely 

 connected with the origin of the chalcedony veins described 

 above. 



DEVIL'S CORKSCREWS (Daemoneliw) 



Among the interesting materials of the badland de- 

 posits few have given rise to more speculations as to their 

 origin than what are known as the Devil's Corkscrews of 

 the Harrison beds. Devil's Corkscrews, or Daemonelix, as 

 they are technically called, have been known by the early 

 residents of northwestern Nebraska for many years but it 

 was not until 1891 when Prof. Barbour made a collecting 

 trip to Harrison and the Badlands that these strange ob- 

 jects were brought to the attention of scientific men. What 

 they really represent or how they were formed is still a 

 matter of conjecture. The more typical forms are upright 

 tapering spirals and they twist to the right or to the left 

 indiscriminately. The spiral sometimes encloses a cylin- 

 drical body known as the axis but it is more often without 

 the axis. Sometimes the spiral ends abruptly below but 

 more often there projects from the lower part one or two 

 obliquely ascending bodies placed much as the rhizomes of 

 certain plants. The size of the well developed form varies 



