260 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



witMn fifty or sixty miles of the Mississippi Eiver in ISTortliern 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 

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

 upon the Paleozoic rocks, the Dakota Groux) being absent there. In 

 Western Iowa and Eastern Nebraska the strata of the Dakota Grouj) 

 are found to rest upon the Paleozoic rocks, the former extending far- 

 ther eastward then than any other Cretaceous strata ; but the eastern 

 border of the Fort Benton and Niobrara Groups are there not 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 i)revailing debris of the plains. 



From the foregoing facts the following inferences may be legitimately 

 di-awn. During the period rei>resented by those Western rocks which 

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

 portion of the Permian period also), the western shore-line of the eastern 

 or principal continental factor extended so far westward that the 

 eastern border of the deposits of the period referred to reached no far- 

 ther eastward than along some line noAv far out on the great plains but 

 the location of which is not known. It is now covered from possible 

 discovery by superimposed Mesozoic strata and the prevailing surface 

 debris. At the close of the Jurassic period a subsidence took place which 

 carried the deposits c€ the Dakota Group nearly as far eastward as Cen- 

 tral Iowa. Still later, continited 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 west- 

 ern i)ortion of the eastern or principal continental factor was resumed, 

 and apparently continued without further interrui)tion by any other sub- 

 sidence sufficient to carry any of the recovered or added land surface 

 again beneath the level of the sea ; although portions of the area which 

 the inter-continental Mesozoic sea had covered were afterward occui)ied 

 by great bodies of brackish and fresh waters. The eastern border of the 

 later Cretaceous deposits was thus carried westward where its place is 

 now covered like that of the earlier border of the Jura-Trias deposits, 

 but not so deeply. 



The eastern border of the Laramie Group is hidden in the same man- 

 ner, but there is yet no evidence that it is anywhere o^'erlapped by any 

 subsequent marine deposit, although it is known to have receiA'ed upon it 

 in several places different groups of fresh-water strata. Perhaps no fact 

 in the physical history of North America is better established than that 

 the elevation of the Eocky Mountains, as such, is of later date than that 

 of the Laramie Group, but the foregoing facts show that both oscillatory 

 movements and general continental elevation took place before the be- 

 ginning of those movements which resulted in the elevation of those 

 mountaius. Besides the oscillations of surface which have already been 

 mentioned, there are indications that other similar movements occurred 

 elsewhere within the same limits of time; such, for example, as the un- 

 conformity of the Laramie strata upon those of the Fox Hills Group in 

 Middle Park, reported by Mr. Marvine ; the unconformity in some places 

 of the Jura-Trias upon rocks older than the Carboniferous, &c. 



But leaving now the subject of the elevation and subsidence of land 

 surface to be resumed further on, the prevailing physical conditions of 

 what is now Western North America may now be considered. No fresh- 

 water deposits of any kind have yet been discovered in any of the Paleo- 



