Page:The Journal of geology (IA journalofgeology11893univers).pdf/119
Appalachian Palæozoics thin out going west until at the Mississippi river they are only 2000 to 4000 feet. The Palæozoics which in the Wasatch are 30,000 feet thin out eastward until they are only 1000 feet on the plains. It follows then that mountains are lines of exceptionally thick sediments.
2. Coarseness of Mountain Sediments.—Mountains are composed mainly of grits, sandstones, and shales, i.e., of mechanical sediments, and most conspicuously so along their axial regions. As we go from this region, sometimes in either direction, but especially in one direction, the strata become finer and finer; sandstones giving way to shales and shales to limestones, i.e., mechanical to organic sediments. This is conspicuously true of the Appalachian; in so many ways a typical mountain. As we pass from the eastern ridge westward, grits and sandstones are replaced by shales and these by limestones. Therefore mountains are also lines of exceptionally coarse sediments.
3. Folded Structure of Mountains.—The folded structure of mountains is perhaps the most universal, and certainly the most significant, of all their features. But there is great variety in the degree and complexity of the foldings. Sometimes the mountain rises as one great fold. The Uinta is an example of this. Sometimes and oftener there are several open folds, like waves of the sea. The Jura is a good example of this. Sometimes and oftenest of all, there are many closely appressed folds. This is the case in the Coast Range of California, in the Appalachian, in the Alps, and probably in the Sierra. The Appalachian may be taken again as the type. In this range the folds are most numerous and most closely appressed in the axial region, and open out and die away in gentle waves as we go westward. Finally, sometimes in extreme cases, as in the Alps, the Pyrenees and probably the Sierra, the strata of the lateral slopes are thrust in under the central and higher parts, so that the strata of these central parts are overfolded outwards on one or both sides. This is the Fan-structure, so marked in the Alps and Pyrenees, where the underthrust and overfold are on both sides, but found also in the Appalachian and Sierra, where they are on one side only.