SOUTH DAKOTA SCHOOL OF MINES 45 



tions of the Nebraska- Wyoming line west of Harrison. 

 (Figure 15). 



The Rosebud Beds. The Arikaree has been studied 

 with much care near Porcupine Butte and farther east on 

 White river by representatives of the American Museum of 

 Natural History. Matthew and Gidley, who first collected 

 fossils there, designated the series of strata as the Rosebud 

 beds. These beds are believed to be approximately equi- 

 valent to the Arikaree formation as the latter is now 

 coming to be understood, but exact relations have not yet 

 been fully determined over any very large section of the 

 country. Matthew describes the beds in their typical 

 eastern locality as follows: "The western part of the 

 formation attains a thickness estimated at 500 feet on Por- 

 cupine creek, a southern tributary of White river. The base 

 is taken at a heavy white stratum which appears to be 

 identical with the stratum capping the White River for- 

 mation on Sheep Mountain in the Big Badlands. This 

 stratum can be seen extending interruptedly across the river 

 to Sheep Mountain, about twenty miles distant, capping 

 several intervening buttes and projecting points of the 

 underlying formation. The Rosebud beds at the bottom 

 approximate the rather hard clays of the upper Leptauchenia 

 beds, but become progressively softer and sandier towards 

 the top, and are capped at Porcupine Butte by a layer of 

 hard quarzitic sandstone. Several white flinty, calcareous 

 layers cover the beds, one of which, about half way up, was 

 used to divide them into Upper and Lower. The strati- 

 fication is very variable and inconstant, lenses and beds of 

 soft fine-grained sandstone and harder and softer clayey 

 layers alternating with frequent channels filled with sand- 

 stones and mud-conglomerates, all very irregular and of 

 limited extent. The hard calcareous layers are more con- 

 stant. A bed of volcanic ash lies near the top of the for- 

 mation, and there may be a considerable percentage of vol- 

 canic material in some of the layers further down. These 

 volcanic ash beds should in theory be of wide extent, and 

 may be of considerable use in the correlation of the scattered 

 exposures on the heads of the different creeks — a very dif- 

 ficult matter without their aid. 



