520 PROCEEDINGS, OF COLUMBUS MEETING. 



stratum of this region of the Plains, so that wells piercing it never go through it. 

 As with the marl, erosion has left sand behind, which aids the formation of dunes. 

 I have called these terranes the Tertiary grit. 



The bottom of the Neocene formation — White River or Loup Fork — rests on a 

 much eroded surface of Mesozoic strata. This pre-Neocene erosion is shown, as well 

 as the later one, in all the deeper valleys of the Plains, and it is manifest that the 

 two succeeding erosions have largely cut down the valleys on the old pre-Neocene 

 lines. 



This being true, it is also true that some of the pre-Neocene and mid-Neocene 

 valleys have not been reopened by modern erosion. They are to be traced by 

 lines of basin-like depressions, and in Nebraska there an 1 examples of modern 

 erosion having cut them transversely. 



When beneath the Tertiaries we examine the subjacent Mesozoic formations; 

 we find a thickening of them toward the north and northwest. This is what we 

 note in the Tertiaries. "Whether this is due to original deposition or to the pre- 

 Neocene erosion, or to both, cannot lie stated certainly; but it is a fact that from 

 Platte river southward on the 102d or 100th meridian the outcrops of the Mesozoic 

 strata in the river valleys are in descending order. Thus, on the 100th meridian 

 we have in — 



Republican valley — Montana shales. 



Sappa " Colorado group — Niobrara. 



Prairie Dog 



Solomon " " " (lower part). 



Smoky 



Walnut " " " Benton. 



Saw Log " " " Benton resting on Dakota. 



Arkansas " not shown. 



( 'rooked creek " Trinity. 



Cimarron " Red beds. 



The valley of the Canadian and Red rivers in the Panhandle of Texas, with a 

 total section of 1,000 feet, shows nothing higher than the Trinity (at least in this 

 longitude). This Cretaceous deposit is, as farther northward, overlain by the 

 Tertiaries. 



< )n the 102d meridian erosion has not proceeded so far and the outcrops are fewer. 

 A little east of it we have in — 



Republican valley — Montana shales. 



Smoky " no outcrop. 



Whitewoman " Colorado — Niobrara (very slight outcrop). 



Arkansas " " Benton. 



Bear creek " Dakota. 



Cimarron " Trinity. 



This relation is represented graphically in figure 7. Reduced to scale for the 

 known elevation, this diagram would show undulations of the strata that can only 

 at present be taken as approximate to the reality till we have a more complete sur- 

 face survey. It is not meant to affirm that the Dakota rests on the Trinity, but on 

 the 102d meridian it is the next southerly outcrop. Further eastward it is known 

 that shell-beds with Gryphea, Turretelki, etc lie above the Trinity sands. 



