SINCLAIR— THE " TURTLE-OREODON LAYER." 463 



pinkish-gray clay full of rolled clay pellets was laid down in layers, 

 apparently at times when the plain was inundated by successive, per- 

 haps very shallow and temporary, floods, a conclusion similar to 

 that reached years ago by Hatcher.* The spreading of such floods 

 would account for the distribution of rolled clay pellets, the over- 

 turning of the shells of dead land turtles, the drowning of others in 

 vast numbers and perhaps the suspension of their gas-distended 

 bodies long enough for decomposition to separate the heads and 

 limbs, for the covering of sun-dried shrunken carcasses and such 

 bones as may have lain on the surface and for the filling of all the 

 cavities in the skulls and turtle shells with fine mud. That such 

 flooding was temporary may be inferred from the widely distributed 

 carnivore excrements, perhaps coated with a film of clay, but still 

 bearing sharp impress of the sphincter ani muscles and certainly 

 not voided in water. 



The brown rusty nodules have, I think, been produced by the 

 rise surfaceward, through capillarity in time of drought, of sub- 

 surface waters charged with carbonate of lime and the deposition of 

 this limey content through evaporation, locally cementing the clays 

 along definite planes parallel to the surface of the ground, forming 

 either solid sheets or bands of nodules, depending, perhaps, on the 

 amount of lime present, the quantity of water being evaporated, or 

 both. Occasionally bones acted as centers of accretion. The rusty 

 stain is a phenomenon confined to the surface of the nodules and 

 was probably produced subsequent to their exposure by weathering. 

 It is not always developed, and the nodules may be similar in color 

 to the investing clays. 



The origin of the sediments of the " turtle-oreodon layer " is part 

 of a larger problem involving the source of the materials of the 

 White River Oligocene as a whole, consideration of which is de- 

 ferred until a later occasion. The harshness of feel which char- 

 acterizes both clays and sandstones is largely due to calcareous 

 impregnation, for, strange to say, while the fossils are in part silici- 

 fied and there is abundant chalcedony in the veins which traverse 

 the clays, the cementing substance of the rusty fossiliferous nodules 



*J. B. Hatcher, "Origin of the Oligocene and Miocene Deposits of the 

 Great Plains," Proc. Am. Phil. Soc, Vol. XLI., pp. 113-131, April, 1902. 



