CLASSIFICATION OF MOUNTAIN RANGES. 669 



tlie Archaean, era, is to-day represented by the Adirondacks, the 

 Laurentide highlands, and the mountains of Labrador, Baffin 

 Land, and Greenland. Asking, then, how the mountain-building 

 forces of eastern North America have been manifested, we see 

 that the upper part of the earth's crust here has been folded by 

 pressure from the Atlantic toward the central area of the conti- 

 nent, exerted during certain epochs of mountain formation, which 

 have alternated with long intervals of repose and of base-leveling 

 by stream erosion. Three chief epochs of orogenic upheaval have 

 produced the intimately blended Laurentian, Atlantic, and Appa- 

 lachian mountain systems, which geologists distinguish because 

 of their diversity in age and in many of their physical features, 

 but which geographers unite as the eastern mountainous belt of 

 our continent. As a whole, it may perhaps properly be called the 

 Appalachian, or, better, the Appalachian-Laurentide belt. 



Other examples of this structure are developed on the grand- 

 est scale in the Old World, comprising the Atlas Mountains, the 

 Pyrenees, the Alps, the Apennines, the Carpathians, the Balkans, 

 the Caucasus, the Elburz, the Hindoo Koosh, and the Himalayas, 

 together reaching from the Pillars of Hercules to the China Sea. 

 These complex mountain systems may collectively be called the 

 Alp-Himalayan belt. During the Miocene, Pliocene, and Glacial 

 periods to the present time, compression has been exerted on each 

 side, upbuilding its mountain chains, which cover a length of 

 about eight thousand miles, occupying a third part of a great cir- 

 cle. In North America the Laurentian mountain system belongs 

 to the remote beginning of the geologic record, and the Atlantic 

 and Appalachian systems are very old, having repeatedly been 

 almost base-leveled ; but these principal mountains of northern 

 Africa and of Europe and Asia are geologically very new, the 

 highest being still in the growth of infancy and youth. When 

 upward growth ceases, erosion triumphs and by slow degrees 

 sweeps the mountain mass into the sea. The perpetuation of an- 

 cient mountain systems has depended on repeated upheavals, and 

 in their present condition they are remnants spared from the ero- 

 sion of areas lately elevated. Portions of this great Eurasian 

 mountain belt began to be plicated and thrust up long before 

 the Tertiary era, and doubtless some of its mountain systems 

 were comparatively undisturbed during the Tertiary and Quater- 

 nary folding and upbuilding of the Alps and Himalayas ; but 

 mainly the prominence of the belt is due to the lateness of the 

 plication, and in part to its being now in progress. 



Nearly all the principal mountain systems of the world have 

 a folded structure, but in many instances they retain no sem- 

 blance of their primal undulations and earliest contour. In the 

 mountainous plateau of Scandinavia and its outlier, the Scottish 



