1. Folded Mountain Ranges.—Foremost in their geographic importance, and in the intricacy and significance of their geologic structure and origin, are the mountain belts which consist of folded rock formations. The strata forming the upper part of the earth's crust are bent up and down in long, nearly straight or curving, wave-like ridges and troughs, and where their disturbance was greatest the successive ridged folds are closely pressed together. The waves of the rock structure are then pushed to such steepness that their sides become parallel with each other, and the entire fold is driven forward into an inclined position. The order of the strata on the lower side of the appressed fold is thus inverted; the originally highest and last formed deposits there lie beneath older beds, in an overturned series. Subaërial erosion then wears down the undulations and the crests of the closely folded strata, often planing them off until a long section, crossing mountain ranges, passes from older to newer beds, and onward from newer to older, in several alternations, having throughout the whole a nearly constant steep dip. Owing to the interbedding of hard and enduring sandstone, quartzite, gneiss, and other rock formations, with more easily eroded limestone, shales, incoherent sandstones, or schists, the erosion commonly produces a new topography, making hollows and long valleys where the more erosible beds have been removed, and leaving ridges and mountain ranges of the harder rocks. More than this, when erosion has been continued through very long periods, it tends toward the ultimate result of removing the upward curved or anticlinal portions of the great folds and sparing the originally lower downward curved or synclinal portions, until valleys take the places which were originally occupied by the highest upheavals, while the original troughs, where the rocks were most compacted by pressure, remain now as the principal mountain ridges. Under denudation, the folded mountainous belt fulfills the prophecy, "Every valley shall be exalted, and every mountain and hill shall be made low."
The most perfect type which the world affords of this structure, or at least the example which has been most fully studied as to the age of its strata, the dates of their foldings and upheavals, and the effects of erosion, is the Appalachian mountain system. As made known by the brothers W. B. and H. D. Rogers and by later geologists, a vast series of Palæozoic strata, representing continuous deposition from the early Cambrian to the close of the Carboniferous period, is thrown into many long, steep folds in the Appalachian ranges of Pennsylvania and the Virginias, making the southeast part of this mountain system, and into plateaus and gentle undulations in the Catskill, Alleghany, and Cumberland Mountains, which are its northern and western portions. After
