logic age, or the epochs of deposition of the rocks forming the mountain range, the epoch of their upheaval or successive upheavals, and the subsequent time during which they have been exposed to erosion.
Six classes of mountain ranges may be discriminated which, from predominant features of their structure, may be named as (1) folded, (2) arched, (3) domed, (4) tilted, (5) erupted, and (6) eroded. In each class the present contour of peaks, ridges, buttresses, and slopes has commonly been produced by the destroying frost, storms, and streams; but wherever some special phase of mountain-building is discoverable within the disguise which the superficial transformation has imposed, the mountain range or separate mountain is thereby referred to its constructive type. The last-named class, therefore, is intended to include only those mountains and ranges which owe their prominence to the denudation of an equal or greater thickness of the same rock formations from the surrounding country or from the valleys that divide them from other mountains, while the structure of the masses spared by this erosion does not place them in either of the five preceding classes as areas that have experienced mountain-building or orogenic movements. The sixth class belongs to areas where continent-building or epirogenic movements of widely extended elevation have been followed by so deep erosion that mountains have been made wholly by sculpture. Often the processes of mountain-building have combined in the same range the features which give names to two or more of these classes, but usually there is some chief element in such complex structure, predominantly allying the range with one of the five orogenic types. Faults, as the geologist calls dislocations of the rock formations, where the portion of the earth's crust on one side of a plane of shearing has been borne upward or forward, while the portion on the other side has fallen downward or backward, often complicate each of the six classes of mountain structure; and they are the sole or principal means of formation of the fourth, that is, of tilted ranges.
The plan of this essay is to examine the structure of each class, and to inquire what was the manner of action of the mountain-building or orogenic forces producing each of the five constructive types, and of the continent-building or epirogenic forces producing the broad, elevated expanses from which erosion has formed the sixth type, mountain remnants of destroyed high lands. Examples of each class are described, and the geologic age of their rock formations, of their upheaval, and of the ensuing erosion, is stated so far as it has been studied out, giving thus, in a brief way, both a description and a history of the mountain range or system.
