308 E. O. ULRlCli REVISION OF THE PALEOZOIC SYSTEMS 



Devonian and Silurian overlaps, is usually underlain by a thin, more or 

 less phosphatic sandstone. The siliceous elastics of this sandstone could 

 not have been derived from the Nashville dome, since none of its surface 

 rocks were capable of supplying such material. Its character is such 

 that only the Ozarkian land to the northwest, where the Saint Peter and 

 older sandstones were locally under tribute in Devonian time, is sug- 

 gested as the probable source. Besides, the elastics derived from the 

 Nashville dome during this long period of emergence, as is positively 

 indicated by the exceedingly slight erosion of the Fernvale and Niagaran 

 rocks, must have been almost negligible in quantity. 



INSTANCE OF SLIGHT EROSION IN THE BIG HORN MOUNTAINS 



Perhaps an even more convincing instance of slight erosion during long 

 periods of emergence and non-deposition than those already cited is the 

 contact of middle or late Mohawkian deposits and ui)per Cambrian in the 

 Big Horn Mountains of Wyoming. None of the elsewhere intervening 

 Ozarkian, Canadian, and early Ordovician formations being well re]> 

 resented in any of the Eocky Mountain areas east of the Great Basin, 

 it seems out of the question to assume that they were deposited there 

 and subsequently removed prior to the great Ordovician transgression. 

 The trend of all lines of evidence respecting these intervening deposits 

 is that their respective seas never covered much of the Eocky Mountain 

 area. The late Cambrian sea, on the contrary, is well known to have 

 transgressed widely in the far and middle west. Hence, according to 

 collateral evidence, upper Cambrian deposits are to be expected in the 

 Big Horn Mountains, while on the same kind of evidence the absence of 

 the other formations is justly ascribed to non-deposition. Following 

 this conclusion, the important fact concerning the contact referred to is 

 that the top of the Cambrian in the Big Horn Mountains is strictly cor- 

 relatable with the upper beds of the Cambrian in Missouri, Oklahoma, 

 Texas, and the upper Mississippi Valle}-, where the zone is overlain by 

 Ozarkian or Canadian deposits. The real significance of this fact is 

 appreciated when we remember that the limestone and dolomite forma- 

 tions deposited elsewhere in the time represented by the hiatus between 

 the upper Cambrian and the Mohawkian in the Big Horn Mountains 

 have an aggregate maximum thickness of over 12,000 feet. 



LATE ORDOVICIAN EROSION IN THE MISSISSIPPI VALLEY 



Perhaps the greatest pre-Pennsylvanian erosion of interior Paleozoic 

 formations was in the closing stages of the Ordovician. South of Hanni- 

 bal. Missouri — in the southern half of the Mississippi Yalley proper — it 



