462 SINCLAIR— THE " TURTLE-OREODON LAYER." 



before interment to the condition of tooth-bearing jaw- fragments, 

 the portion of the body most resistant to decay. Turtle shells and 

 all the cavities in the skulls are completely filled with a harsh-feeling 

 pinkish-gray^ clay full of small rolled clay pellets. These are not 

 concentrated into lenses, but evenly distributed throughout the clay. 

 Mouse-nibbled bones and abundant undissolved coprolites of car- 

 nivores are common features in the same matrix with the skulls and 

 turtle shells. One associated Oreodon skeleton collected by the 

 Princeton Expedition of 1920 had the head and neck bent back in a 

 manner frequently observable in the more or less dried-up carcasses 

 of sheep killed by winter storms. A film of iron oxide, red or 

 yellow, frequently covers the fossil bones, their organic content per- 

 haps acting as a precipitant on iron compounds which subsequently 

 oxidized. Some of the original calcareous material of the bones 

 still remains, but much has been replaced by silica, which may com- 

 pletely fill marrow cavities in limb bones, and pulp canals in teeth, 

 and oqcurs also in the form of chalcedony veins in the numerous 

 shrinkage cracks which traverse both Titanotherium and Oreodon 

 beds. 



Turning now to stratigraphic and lithologic data, in the basin of 

 Indian Creek, when viewed from a distance, the Chadron-Brule con- 

 tact is seen to be remarkably even and free from sinuosities, appar- 

 ently a base-leveled erosion plane separating the soft cross-bedded 

 clays and channel sandstones of the Titanotherium beds, with their 

 smooth hummocky hill profiles, from the horizontally stratified, 

 evenly color-banded, harsh-feeling Oreodon clays with their den- 

 dritic type of dissection and steeper slopes. The sudden disap- 

 pearance of titanotheres at the contact is further proof of a strati- 

 graphic break. When examined close at hand, the contact loses 

 much of its distinctness, owing to the softness and similarity of ma- 

 terials on either side of the erosion plane. Over this surface, on 

 which were numerous minor ponds floored with marl (the silicified 

 limestones already described as occurring at the contact), a harsh 



3 The color is about intermediate between " seashell pink " and " pale och- 

 reous-salmon " of Ridgway's Color Standards and Nomenclature, Pis. XIV., 

 XV., 1912, with a pale gray tone and has accordingly been spoken of above 

 as pinkish-gray. 



