464 SINCLAIR— THE " TURTLE-OREODON LAYER." 



and of the soft channel sandstones is carbonate of hme and not 

 silica. Only rarely does the matrix of a fossil or the immediate 

 vicinity of a large chalcedony vein show siliceous impregnation. 

 When the calcareous cement of the channel sandstones associated 

 with the " turtle-oreodon layer " is dissolved in hydrochloric acid, 

 the insoluble residues are found to abound in quartz grains, foils of 

 white mica, clay pellets and fine clay dust, and very finely divided 

 pinkish clay with minute quartz fragments constitutes the matrix of 

 the " red layer " itself. While I have found abundant volcanic ash 

 in one of the higher horizons of the Oligocene, I have not estab- 

 lished its presence in the " turtle-oreodon layer," although it is per- 

 fectly possible that it occurs mixed with the pinkish clays and per- 

 haps may have supplied the silica which replaces fossils and fresh- 

 water limestones and occurs in such abundance in the chalcedony 

 veins. The latter are posterior in origin to the caliche nodules in 

 the upper part of the " red layer," since fossils and nodules are 

 frequently cut by them. Rosettes and geodes are other forms as- 

 sumed by the same material. 



I am of the opinion that a climatic factor is involved in the 

 problem of the origin of the White River sediments, but, at the 

 present stage of the investigation, am not prepared to demonstrate it 

 fully. Certain facts observed in connection with the horizon we 

 have been discussing seem to bear on the subject. In the valley of 

 Indian Creek the Titanotherium beds rest with marked unconformity 

 on the eroded surface of the Pierre shale, with a basal conglomerate 

 at the contact containing pebbles of quartz, chert and other hard 

 rocks. The top of the substratum has considerable topographic re- 

 lief, so that the thickness of the Titanotherium beds varies from 

 place to place. Throughout this formation cross-bedding is com- 

 mon and there are frequent large channels filled with coarse quartz- 

 ose sediments. The impression conveyed is that of fluviatile deposi- 

 tion under conditions of generous rainfall, abundant run-ofif and 

 rapid sedimentation. Accumulation of detritus eventually ceased, 

 temporarily, and erosion began, producing the base-leveled surface 

 which separates the Titanotherium from the Oreodon beds, an event 

 which I would associate with a climatic rhythm, perhaps a swing 



