South Dakota School of Mines - 25 



cene and Pliocene, the Miocene (or Lower Miocene) being 

 often referred to as the- White River group. Later as the methods 

 of correlation became more refined and as representative fossils 

 came more abundantly and in better condition from the hands of 

 the collectors, giving better opportunity for comparison with sim- 

 ilar fossils in other parts of the world, the lower beds were found 

 to be equivalent to the Oligocene and the upper beds to the Mio- 

 cene, chiefly Lower Miocene, the Oligocene being in many ways 

 the more important. This is now the accepted correlation. 

 Pliocene deposits are known to occur widely distributed in west- 

 ern Nebraska and thin sheets or outliers extend northward into 

 the area under discussion. They have not been fully studied, 

 but it is believed that they can not be of great lithologic im- 

 portance.* 



An important work of investigators has been to further 

 sub-divide the deposits and to correlate in so far as possible the 

 resulting subdivisions. Hayden early attempted a subdivision 

 and with marked success so far as materials then at hand would 

 allow. A complete restatement here of his section is perhaps 

 not desirable, but a brief reference to it may serve a good pur- 

 pose. Six beds were recognized. The oldest was designated 

 as Titanotherium Bed A, and next above this was Turtle and 

 Oreodon Bed B. Continuing in ascending order were Bed C, 

 Bed D, Bed E, and Bed F. Titanotherium Bed A, now known 

 in the literature as the Chadron formation or the Titanotherium 

 zone, stands practically as designated more than fifty years ago 

 by the author of the term. The Turtle and Oreodon Bed B, as 

 outlined, has withstood the scrutiny of later investigators nearly 

 as well as the lower bed, although a slightly different definition 

 as to what shall be included has been found advisable. The beds 

 above this are less uniform in character and their correlation 

 has been the subject of much more discussion. 



The present classification, shorn of some local and conflict- 

 ing peculiarities, is given in the table on the following page. 

 There seems little need for the purpose of this paper, in view of 

 the fact that the various formations are individually described 

 under Character of Deposits, to further detail the successive 



*For recent discussion of this subject the reader is referred to 

 the following papers: Cenozoic Mammal Horizons of Western North 

 America by Oisborn, with Fauna! Lists of the Tertiary Mammalia of 

 the West by Matthew, IT. S. Geol. Surv., Bull. 361, 1909; and A 

 Pliocene Fauna from Western Nebraska ,by W. D. Matthew and Harold 

 J. Cook, Am. Mutf. Nat. Hist., Bull. Vol. 26, pp. 361-414, 190'. 



