ORIGIN OF MOUNTAIN RANGES. 545 



Appalachian Palaeozoics thin out going west until at the Missis- 

 sippi river they are only 2000 to 4000 feet. The Palaeozoics 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 

 li?ies of exceptionally thick sediments. 



2. Coarseness of Mountain Sediments. — Mountains are composed 

 mainly of grits, sandstones, and shales, i.e., of mechanical sedi- 

 ments, and most conspicuously so along their axial regions. As 

 we go from this region, sometimes in either direction, but espe- 

 cially in one direction, the strata become finer and finer; sand- 

 stones giving way to shales and shales to limestones, i.e., mechan- 

 ical 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. Some- 

 times and oftener there are several open folds, like waves of the 

 sea. The Jura is a good example of this. Sometimes and often- 

 est 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, some- 

 times 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 under- 

 thrust and overfold are on both sides, but found also in the 

 Appalachian and Sierra, where they are on one side only. 



