666 THE POPULAR SCIENCE MONTHLY. 



logic age, or the epochs of deposition of the rocks forming the 

 mountain range, the epoch of their upheaval or successive up- 

 heavals, 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, but- 

 tresses, and slopes has commonly been produced by the destroy- 

 ing 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 denuda- 

 tion 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-build- 

 ing or orogenic movements. The sixth class belongs to areas 

 where continent-building or epirogenic movements of widely ex- 

 tended elevation have been followed by so deep erosion that 

 mountains have been made wholly by sculpture. Often the pro- 

 cesses 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 ensu- 

 ing 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. 



