180 Professor H. D. Rogers^ on the Geology and [Feb. 8, 



and tertiary deposits that fringe them, was that well-marked physical 

 limit already partially traced as the inner edge of the low tertiary 

 and cretaceous plain. Probably, however, this newly-produced part 

 of the continent, particularly on its western side, was somewhat more 

 extended at the date of elevation than the present margin of the 

 cretaceous formation indicates; for it is upon this supposition, 

 coupled with a belief that the newly uplifted formations remained 

 out of water, under somewhat wider boundaries than they now 

 exhibit, that we can best explain the non-existence of any Permian 

 and Triassic formations between the Carboniferous, the latest of the 

 American palaeozoics, and the Cretaceous, the next more recent 

 sediments deposited against them. During all the long geological 

 ages which intervened between the lifting out the palaeozoic region 

 of the United States east of the Missouri, and the deposition of the 

 Cretaceous strata, no sedimentary formations of the Mesozoic periods, 

 such as those which fill large tracts in other quarters of the globe, 

 were permanently upraised into dry land, saving only a few narrow 

 strips — products of estuary sedimentation — stretching at intervals 

 along the Atlantic slope from Prince Edward's Island to Carolina. 

 Elsewhere, certainly as far westward as the Rocky Mountains, either 

 nothing was deposited during the permian, triassic, and Jurassic ages, 

 or, what is far more probable, the formations tlien produced were 

 formed outside of the present palaeozoic limits, and have been 

 covered up from sight by the wider sediments of the cretaceous sea, 

 which, lapping over them, have shut in all the earliest border tracts 

 of the palaeozoic lands, formed at the end of the coal period. 



The nature of the crust movements which elevated the palaeozoic 

 strata was in the region of maximum disturbance— that of the 

 Atlantic slope and Appalachian chain, — a stupendous undulation 

 or wave-like pulsation, the strata being elevated into permanent 

 anticlinal and synclinal flexures, remarkable for their wave-like 

 parallelism, and for their steady declining gradation of curvature, 

 when they are compared in any east and west section across the cor- 

 rugated zone. To the westward of the Appalachian chain, where 

 this structure is so conspicuous, the crust waves flatten out, recede 

 from each other, and vanish into general horizontality ; and this 

 nearly level condition extends thence throughout all the older rocks 

 to the plains of Texas and Nebraska, where the cretaceous beds 

 overlap them. The orographic features of the Appalachian chain, 

 even to the minutest slopes and terraces upon the flanks of the 

 ridges, are all beautifully impressive of the carving action of deep 

 retreating waters. 



The cretaceous deposits of the United States all imply a marine 

 origin, no fresh water remains having hitherto been discovered 

 among them ; and the geological map now exhibited pictures ap- 

 proximately the wide extent of the cretaceous, or later mesozoic 

 sea, as it washed the boundaries of the palaeozoic continent. The 

 shore of that sea was, as before hinted, the inner edge of the Atlantic 



