Mountains by folding are generally of moderate elevation, while
mountains by fracture include the highest chains of the globe. The
Appalachian Mountains in North America, and the Jura in Europe, are
examples of the first; the Rocky Mountains, Andes, Alps and Himalayas,
of the second.
Folded mountains are curved into long arches, either entire or broken at
the summit and forming a system of long, parallel ridges, of nearly
equal height, separated by trough-like valleys. Here and there,
however, deep gaps, or gorges, cut the chains allowing the rivers to
escape from one valley to another.
In systems of mountains produced by fracture, there is usually one main
central chain, with several subordinate ranges. They have, however, less
regularity and similarity among themselves than the parallel chains of
mountains by folding.
The crests are deeply indented, cut down one-third or one-half the
height of the range, forming isolated peaks and passes which present to
the eye the appearance of a saw, called in Spanish Sierra; in
Portuguese, Serra. Such ranges are frequently distinguished by these
terms, as the Sierra Nevada, in North America; and the Serra do Mar, in
Brazil.