796 



DYNAMICAL GEOLOGY. 



tive beds, all conformable to one another. The next mountain-mak- 

 ing of the summit region followed the close of the Miocene. Finally, 

 the whole Rocky Mountain region, for a breadth of more than a thou- 

 sand miles, was undergoing elevation during the Tertiary, and did not 

 reach its ultimate height before the beginning of the Quaternary. 



It is thus apparent that mountain-making has taken place after im- 

 mensely long periods of quiet and of gentle oscillations, all of which 

 were periods of gradual preparation for the great event. 



3. Repetitions of the Event in the same Great Continen- 

 tal Region, or on the same Continental Border. 



The facts just stated illustrate the repetitions that have occurred on 

 the Atlantic and Pacific borders of North America. 



Another important truth connected with the successive events is 

 this: that the resulting ranges have a general parallelism to one an- 

 other, and are parallel to the outlines of the original V-shaped Ar- 

 chaean dry land, as explained on page 1 60. Even the great bends in 

 the present course of the Atlantic border are repeated in those of 

 the Triassico- Jurassic areas ; and in those of the Appalachian range, 

 whose northern part has the trend of Long Island and southern New 

 England, and the southern, that of the Atlantic coast farther south ; 

 and, more to the north, in the course of the Green Mountains, which 

 corresponds to the Adirondacks, but trends more northeasterly on the 

 northern borders of New England. The repetitions were hence not 

 only in the event, but in its chief characteristics. 



4. Results. 



From these mountain-making events, and their repetitions in the 

 same wide continental region, have come the great mountain masses 

 of the globe, and even the Continents, which are the combined results. 

 The following kinds of mountain masses are to be distinguished : — 



1. Individual Mountain Ranges. — Monogenetic and Polygenetio 

 Mountains. — The Appalachians, a range of many mountain ridges, 

 and valleys, constitute one individual among mountains, because a re- 

 sult of one genetic process, or, in a word, monogenetic. An individual 

 range thus embraces all the ridges or elevations, or the whole area of 

 comformable rocks, that received their flexures or upturning in the 

 same mountain-making epoch. The Green Mountains are an exam- 

 ple of another monogenetic or individual range. The Adirondacks, 

 the Highland Range (of New Jersey and New York) and its continu- 

 ation southward, with whatever Archasan areas to the northward were 

 cotemporaneously upturned, probably constitute another monogenetic 

 range. The Mesozoic trap and sandstone areas are parts of another 

 line. 



