Page:The Journal of geology (IA journalofgeology21894univers).pdf/47
by which the level is determined, and gradually spreads inland toward the principal divides. Under similar conditions the shales and limestones wear away more rapidly than the coarser sediments and crystalline rocks, and local baselevels appear for a time determined by the harder rocks. But these are all obliterated in a general baselevel when it is completely developed. The land is so unsteady that it rarely, if ever, remains without elevation or depression long enough for the complete development of a baselevel of erosion. It commonly happens, however, that the large masses of harder rocks upon the slopes of the principal divides form independent elevations in the plain which may be more or less distinctly defined upon the softer rocks. The topography of the region is then essentially a peneplain.
It is evident that a general baselevel of erosion must have originated approximately at sea level. This is the only position in which a very extensive baselevel of erosion can originate. If we now find such a baselevel at considerable elevation above the sea, its position furnishes evidence that since the baselevel was formed the country has been uplifted in the process of mountain building.
Upon our Atlantic slope, ancient baselevels of erosion are well developed in the Piedmont region and elsewhere at considerable altitudes above the sea, as shown by Davis, McGee, Willis, Hayes, and Campbell. The ancient mountains have been swept away, and the modern mountains, at least in large part, are the result of later upheavals. Similar changes have taken place on the Pacific slope. Russell found in the St. Elias range, at an elevation of over 5,000 feet, shells of marine mollusks still living along the Pacific coast, showing that the great mountain range had been uplifted in very late geologic time. So, also, the Sierra Nevada and Coast ranges, and to some extent the Cascade range, now such prominent features of the Pacific coast, have been upheaved to their present great height, and deep cañons cut upon their slopes in the later geologic ages. At an earlier epoch the whole country was comparatively low and near sea level, or, in other words, near its baselevel of erosion. The mountain