190 WANLESS— LITHOLOGY OF WHITE RIVER SEDIMENTS. 



Sheep Mountain in the Big Badlands. They are subdivided in ascend- 

 ing order into : 



(a) Vertical, columnar weathering, ash beds often affording abun- 



dant nodules somewhat resembling coprolites in shape and size. 

 Each nodule has a vertical central tube around which the 

 nodule seems to have grown by concentric accretion. The 

 cementing material of these nodules has not yet been investi- 

 gated. The general color of this division is pale pink. Thick- 

 ness, 165 feet. 



(b) A massive, white, volcanic ash layer at the top of the zone of 



columnar weathering, 66 feet thick. This and division (a) are 

 shown in Plate II., Fig. i. This zone is more resistant to 

 weathering than (a), but does not show vertical faces. 



(c) A capping of 50 to 75 feet of pinkish nodule-bearing volcanic 



ash beds, exposed on the highest parts of Sheep Mountain. 



This gives a total thickness of 281-316 feet for the Sheep Moun- 

 tain Leptauchenia beds section. If Matthew^ is correct in correlating 

 the Sheep Mountain " White Ash " layer with that exposed in the 

 Porcupine Creek section of the Lower Rosebud Miocene, about 20 

 miles south of Sheep Mountain, divisions (b) and (c) of the above 

 sequence may belong rather to the Lower Rosebud than the White 

 River. This correlation is at present tentative and must be worked 

 out by a more thorough study of both the stratigraphy and the faunal 

 changes. 



Additional stratigraphic details, it is hoped, may be presented in 

 a subsequent paper. 



III. Method of Investigation. 



The method pursued in the examination of the sediments is essen- 

 tially that described by Crook.'^ A sample of 50 grams was selected 

 for study and, in the case of the finer sediments, was crushed so 

 that all of it passed through a one-millimeter mesh. This was done 

 in order to allow the acid to act freely on all parts of the sediment, 

 so that all of the calcareous cement would be dissolved. This was 



6 Matthew, Bull. Am. Mus. Nat. Hist., Vol. 23, pp. 169-219, 1907. 



7 Hatch & Rastall, "Textbook of Petrology: The Sedimentary Rocks,"' 

 1913. pp. 335^414- 



