“While mountains and mountain chains all over the world, and low lands, also, have undergone uplifts, in the course of their long history, that are not explained on the idea that all mountain elevating is simply what may come from plication or crushing, the component parts of mountain chains, or those simple mountains or mountain ranges that are the product of one process of making—may have received, at the time of their original making, no elevation beyond that resulting from plication.
This leads us to a grand distinction in orography, hitherto neglected, which is fundamental and of the highest interest in dynamical geology; a distinction between—
1. A simple or individual mountain mass or range, which is the result of one process of making, like an individual in any process of evolution, and which may be distinguished as a monogenetic range, being one in genesis; and
2. A composite or polygenetic range or chain, made up of two or more monogenetic ranges combined.
The Appalachian chain—the mountain region along the Atlantic border of North America—is a polygenetic chain; it consists, like the Rocky and other mountain chains, of several monogenetic ranges, the more important of which are: 1. The Highland range (including the Blue Ridge or parts of it, and the Adirondacks also, if these belong to the same process of making) pre-Silurian in formation; 2. The Green Mountain range, in western New England and eastern New York, completed essentially after the Lower Silurian era or during its closing period; 3. The Alleghany range, extending from southern New York southwestward to Alabama, and completed immediately after the Carboniferous age.
The making of the Alleghany range was carried forward at first through a long-continued subsidence—a geosynclinal (not a true synclinal, since the rocks of the bending crust may have had in them many true or simple synclinals as well as anticlinals), and a consequent accumulation of sediments, which occupied the whole of Paleozoic time; and it was completed, finally, in great breakings, faultings and foldings or plications of the strata, along with other results of disturbance.
These examples exhibit the characteristics of a large class of mountain masses or ranges. A geosynclinal accompanied by sedimentary depositions, and ending in a catastrophe of plications and solidification, are the essential steps, while metamorphism and igneous ejections are incidental results. The process is one that produces final stability in the mass and its annexation generally to the more stable part of the continent, though not stable against future oscillations of level of wider range, nor against denudation.
It is apparent that in such a process of formation elevation by direct uplift of the underlying crust has no necessary place. The attending plications may make elevations on a vast scale and so also may the shoves upward along the lines of fracture, and crushing may sometimes add to the effect; but elevation from an upward movement of the downward bent crust is only an incidental concomitant, if it occur at all.
We perceive thus where the truth lies in Professor Le Conte’s important principle. It should have in view alone monogenetic mountains and these only at the time of their making. It will then read, plication and shovings along fractures being made more prominent than crushing:
Plication, shoving along fractures and crushing are the true sources of the elevation that takes place during the making of geosynclinal monogenetic mountains.