THE THROES OF THE CONTINENT. 267 



Great changes now took place. The Nevada continent, 

 which had yielded thousands of cubic miles of Palaeozoic 

 sediments, now, in its turn, went down, and a broad region 

 eastward, as far as the Wahsatch Mountains, came up. The 

 Cordilleran continent was now located in the region at present 

 known as the "Basin Province," embracing Great Salt Lake, 

 Pyramid Lake, and others. Only the foundations of the 

 Sierra Nevada had been laid. The Basin Continent on the 

 east was to be ground up to supply the masonry for a new 

 structure ; just as the Nevadan foundation itself was the 

 mere stump of a land pulverized to supply materials for 

 the Basin Land. So nature's method is to build, demolish, 

 and rebuild. As in the successions of life, so in the succes- 

 sions of continents. 



But while this continental see-saw was in progress west of 

 the Wahsatch, all remained quiet to the east, as far as the 

 Great Plains. In the Appalachian region, however, a similar 

 see-saw occurred. The loaded Appalachian belt came up in a 

 series of mountain folds fifteen thousand feet high, while the 

 Seaboard Land, that had been wasted in supplying the load, 

 went down most of it, like the old Nevadan continent, below 

 sea-level. Simultaneously, the whole breadth of the country 

 westward to the Great Plains, was finally annexed to the 

 eartern limb of North America. 



It was now the beginning of the geologic Middle Ages. 

 East of western Kansas, the land was completed, save the 

 Atlantic and Gulf border. West of the same meridian 

 stretched a broad ocean over the region of the Great Plains ; 

 across the belt of the Rocky Mountains, where it was inter- 

 rupted by the meridionally disposed Colorado, Medicine Bow, 

 and Park Ranges ; across the broad Plateau region of the pres- 

 ent, to the base of the newly uplifted Basin Laud which was 

 now melting away under the sedimentary demands for Meso- 

 zoic materials on the east and the west. To the south, this 

 ocean extended to the Gulf of Mexico. Northward, it joined 

 the Arctic Ocean. West of the Basin continent, the sedi- 

 ments of the Triassic and Jurassic formations were laid down 



