58 THE WHITE RIVER BADLANDS 



adjacent or nearby rock masses without any immediate in- 

 iluence either of heat or of solvent action. These clastic 

 bodies are known as dikes also. 



Many writers have commented upon the nature and 

 abundance of the dikes and veins in the Badlands. Al- 

 though constituting minor features of the landscape they 

 are nevertheless extremely abundant in places and not 

 infrequently they display themselves in an interesting 

 and complicated manner. The dikes are made up generally 

 of a soft greenish sand or sandy clay. This usually wears 

 away a little more readily than the enclosing strata but 

 sometimes it resists weathering better and then the dike 

 projects above the general surface. The prevailing attitude 

 is nearly perpendicular and the dike outcropping in a 

 straight line may occasionally be traced across gulches and 

 draws and over ridge and pinnacle and mound for a mile or 

 more. The thickness is commonly not more than a few 

 inches but it sometimes reaches two or three feet. The 

 dikes are supposed to occupy preexisting cracks, the ma- 

 terial having been forced in from below by hydrostatic 

 pressure or by the weight of the superincumbent strata. 

 It is possible that in some cases the material may have 

 come from above. 



The veins are chiefly chalcedony. They resemble the 

 dikes so far as concerns position and form and, aside from 

 the fact that they were deposited from solution, are believed 

 to have much the same history. They average thinner than 

 the dikes, are much harder, and are in many places more 

 abundant. They resist weathering much better than the 

 enclosing clays, hence commonly present a jagged line above 

 the surface. As the supporting clay becomes loosened and 

 is carried away the thin chalcedony breaks into platy 

 angular fragments and these falling upon the surrounding 

 surface protect it from further erosion much as would a 

 shingle roof. 



Geodes. Geodes are spheroidal masses of mineral mat- 

 ter formed by deposition of crystals from some mineral 

 solution on the walls of a rock cavity. The growth is con- 

 stantly inward toward the center. If the process of deposi- 

 tion has continued sufficiently long, the crystals reach 

 across the depositional space, interlock with each other, and 

 the geode becomes solid. Often the crystals project only 



