Fig. 475.—The interrupted system of the Armorican Mountains common to western Europe and eastern North America (after Arldt).
Thus the mountain folds of the Appalachian system are in Newfoundland cut off abruptly at the coast line, and the same beds, similarly truncated, are encountered again across the expanse of ocean in the folds at the coast of western Europe ([Fig. 475]). In discontinuous remnants this ancient mountain chain may be traced in an east and west direction across western and central Europe. We have thus here to do with a single mountain system which extends from central Europe to northern Alabama, out of which a great link has been taken by the subsequent sinking in of the basin of the Atlantic Ocean.
Fig. 476.—Schematic representation of a “zone of diverse displacement” in the Great Basin of the western United States (after Powell).
The block type of mountain.—The inclusion of most elevations in mountain chains and arcs is one of the most obvious facts to any one who has examined world atlases with this subject in mind. Such chains are almost invariably composed of folded rocks, thus indicating that erosion has removed great superincumbent masses of strata since the crustal compression produced the folds at considerable depths below the then surface.
There are, however, large elevated tracts upon the earth’s surface which are intersected by deep valleys, but where no arrangement of the elevated portions within chains or ranges is to be detected. In such cases the distribution of mountain and valley may bear a resemblance to a mosaic of disturbed parts which stand at different levels ([Fig. 476]).
Fig. 477.—Section of an East African block mountain (after J. W. Gregory).
Such block mountain districts are to be found in many parts of the earth’s surface, but notably within the Great Basin of the western United States, and in the land area which borders the Indian Ocean upon the west and northwest. In contrast with the mountain arcs, so strikingly exemplified by the continent of Asia as a whole, its extreme southwestern portion is made up of an alternation of plateau and rift valley separated from each other by great displacements. Though modified to some extent by erosion, the elevations seem generally to represent the displaced crust blocks which in mutual adjustments have been left at the highest levels ([Fig. 477]). The valley of the Jordan, with the mountains of Lebanon rising above it, is near the northern extremity of this faulted mountain region ([Fig. 434], [p. 404]), while the Great Rift valley, crossing east Central Africa, and the many neighboring rifts to the east and west, are graven in lines so deep that an observer upon a neighboring planet might perhaps detect them.