586 POPULAR SCIENCE MONTHLY. 



period, the accumulations of sucli movements have raised the same 

 superficial formations of the coastal plains of America to an altitude 

 of one hundred feet near Cape Hatteras, to eight hundred feet 

 farther south, to fifteen hundred feet in the neighboring mountain 

 region, to only three hundred or four hundred feet in the Mississippi 

 Valley; while the same coastal plain in Mexico has been elevated to 

 seventeen hundred feet. Farther south, in the Tehuantepec Isthmus, 

 the corresponding change of level has not been more than eight 

 hundred feet. These movements do not bend, crumple, or break 

 the strata, or appreciably affect their horizontality. The amount 

 of elevation just mentioned was sufficient to cause the final separa- 

 tion of the Mexican Gulf and the Pacific Ocean, but inadequate to 

 account for the high plateaus. 



Another class of terrestrial movements gives rise to mountain 

 folds, thrusts, and faults, with a general dislocation and disturbance 

 of all the formations. Just how much of a plateau was elevated 

 by these forces is not apparent, for the surface base levels of erosion 

 still retain their older courses of drainage. Consequently, one is led 

 to suspect that the great escarpments are not entirely due to denuda- 

 tion, but that a third class of earth movements has obtained, lifting 

 the plateaus abruptly to a considerable proportion of their height 

 (six thousand feet, more or less) above the inner margin of the 

 coastal plain, without greater deformation of their surface than that 

 of the coastal plains themselves. This third movement seems to 

 be a sort of squeezing up of great segments of the earth's crust, 

 without tilting its surface by more than a few feet per mile; con- 

 sequently, it imjDlies a great dislocation and slip or fault along the 

 margin of the table-land, since modified upon the surface by the 

 atmospheric denudation. The analysis of these complex movements 

 is far from complete, and, although we do not fully understand them, 

 this ignorance of causes does not affect the evidence of the very recent 

 elevation of the plateaus. 



The Peeiods of Great Elevation.- — The old geological forma- 

 tions of the plateaus are mostly buried beneath secondary accumula- 

 tions; so that the present physical surface features are largely due 

 to the atmospheric agents which have been at work since about the 

 end of the Cretaceous period, as out of the formations of that date 

 a large portion of the base levels of erosion have been molded. 

 These great base levels required a long time for their development, 

 Avhich was j^rovided for during the greater portion of the subsequent 

 Tertiary period, when the present table-lands were mostly low con- 

 tinental plains interrupted by mountain ridges. In the meanwhile, 

 the present coastal plains (now rising to seventeen hundred feet) 

 were submerged and were receiving the older Tertiary accumulations 



