870 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



formations; and as these aggregate a great thickness, there was, of 

 course, for a long time, and over a very large part of the space which it 

 occui)ied, a gradnal subsidence of the bottom which allowed the succes- 

 sive deposition of shallow-water formations. The following facts prove 

 the occurrence of oscillations cf land-surface and sea-bottom by which 

 from time to time the eastern border of the Mesozoic sea was shifted, 

 and the whole finally displaced. 



In Western Iowa, Eastern !N'ebraska, and Eastern Kansas, the Creta- 

 ceous strata are known to rest directly upon the Carboniferous strata, the 

 Jura Trias being absent. These last-named strata, however, are in full 

 force where the Mesozoic rocks are turned up against the eastern flanks of 

 the Eocky Mountains and Black Hills, as well as farther westward. Their 

 eastern border is certainly somewhere in the great plains beneath later 

 Mesozoic formations and the prevailing surface dehris, but its location 

 is not even approximately known. Cretaceous strata continuous with 

 those of the West are known to have been deposited as far eastward as 

 within 50 or 60 miles of the Mississippi Eiver in Northern Iowa and 

 Southern Minnesota ; southward from which region their eastern border 

 gradually recedes to the westward nearly as far as Central Kansas. In 

 the northeastern region just named, it is the attenuated strata of the 

 Eort Benton and Niobrara Groups that are found, and these rest directly 

 upon the Paleozoic rocks, the Dakota Group being absent there. In 

 Western Iowa and Eastern Nebraska, the strata of the Dakota Group are 

 found to rest upon the Paleozoic rocks, the former extending farther 

 eastward there than any other Cretaceous strata ; but the eastern bor- 

 ders of the Fort Benton and Niobrara Groups are not there very far to the 

 westward. The eastern border of the Fort Pierre and Fox Hills Groups, 

 or the Later Cretaceous, is still farther westward, but its position is 

 hidden by the later formations and the prevailing debris of the plains. 



From the foregoing facts, the following inferences may be legitimately 

 drawn : — During the period represented by those Western rocks which 

 have received the designation of Jura Trias (and apparently during a 

 portion of the Permian period also), the western shoreline of the east- 

 ern or principal continental factor was extended so far westward that 

 the eastern border of the deposits of the period referred to reached 

 no farther eastward than along some line now far out on the great 

 plains, but the location of which is not known. It is now covered from 

 l^ossible discovery by superimposed Mesozoic strata and the prevailing 

 surface debris. At the closeof the Jurassic period, a subsidence took i)lace, 

 which carried the deposits of the Dakota Group nearly as far eastward 

 as Central Iowa. Still later, continued subsidence, but of more limited 

 extent to the southeastward, caused the deposition of Fort Benton and 

 Niobrara strata still farther eastward, in Northern Iowa and Southern 

 Minnesota. At or before the close of the Niobrara epoch, the elevation 

 of the western portion of the eastern or principal continental factor was 

 resumed and apparently continued without further interruption by any 

 other subsidence sufficient to carry any of the recovered or added land- 



