South Dakota School of Mines 49 



» 

 are connected with the chalcedony crystals the veins may exist 

 on one or both sides of the intruded material between it and the 

 clay walls of the crack. The absence of the crystals from one 

 side or the other is not a local accident, but seems constant for 

 large areas. In every case where it occurs on one or both sides 

 of the core, the crystals have a perfect vein structure, presenting 

 a flat surface to the core or dike and one to the clay wall, and 

 meeting irregularly in the middle." 



As to the method of formation, Mr. Case concludes : "The 

 dikes of mud and sand occupy pre-existent cracks which were 

 filled by intrusions below by water and suspended material. The 

 water was forced into the cracks from porous layers either by 

 hydrostatic pressure or by that of the superincumbent strata, 

 probably both movements. They are in all probability both mud 

 cracks and cracks formed by seggregation of the clays around 

 local centers. The veins of chalcedony were formed by the en- 

 trance into similar origin as those containing the dikes, of sili- 

 cated waters. The cracks were already filled more or less with 

 water and sand. The thinning out of the seams from above 

 downward indicates that the silicates filtered in from above."* 



In the same year, 1894, Prof. Todd observed a number of 

 dikes along the great wall near Sage creek and "near Black 

 Postofhce in the south part of Ziebach county" on White river. 

 Near Sage creek he found two dikes crossing each other at a 

 small angle running nearly east and west. Across them were 

 two others nearly north and south. They were found to vary 

 n width from six to eight inches. The middle portions were 

 :olumnar jointed, the outer portions perpendicularly fluted and 

 showed other signs of upward motion of the mass. Prof. Todd 

 ~oincides with Prof. Hay in the belief that the fissures have been 

 produced by earthquake disturbances and agrees with both Prof. 

 Hay and Prof. Case in considering that the material was in- 

 truded fro mbelow." 



Xow that the badland formations have been more widely 

 studied, it is known that dikes and veins cut the formations 

 abundantly in many places and an observant visitor will often 

 find them an interesting feature of the country. 



In 1899 I found five or six miles northwest of Sheep Moun- 

 tain, near the northeastern side of Indian draw, a dike of coarse 



*Case, E. C On the Mud and Sand Dikes of the White River 

 Miocene. Am. Geol., Vol. 15, 1895, pp. 248-254. 



tTodd, J. E. A Preliminary Report on the Geology of South 

 Dakota. S. D. Geol. Surv., Bull. Xo. 1, p. 106, 1894. 



