722 DYNAMICAL GEOLOGY. 



phism where the plications were most numerous. Limestone is 

 always in solid layers unless quite impure. 



4. Formation of valleys. — The plication of the earth's crust pro- 

 duces alternating depressions and elevations, unless the folds are 

 pressed together into a close mass. The depressions are synclinal 

 valleys. The minor valleys of this kind are generally obliterated 

 by subsequent denudation ; and often even the summits of ridges, 

 under this latter agency, may consist of the rocks of a synclinal 

 axis. Besides synclinal valleys, there are often also monoclinal valleys 

 (p. 720). In addition, there are wider depressions lying between 

 distant ranges of elevations which were produced through a gentle 

 bending of the earth's crust (made up of plicated strata or not) ; 

 and these great valleys or depressions (like the Mississippi and 

 Connecticut valleys) may be called geoclinal, the inclination on which 

 they depend being in the mass of the crust, and not in its strata. 



5. Elevation of mountains. — The force engaged in producing the 

 great systems of plications over the earth is sufficient for the ele- 

 vation of mountains of all heights. 



Mountain-chains are not made of igneous ejections, except occa- 

 sionally in some small portions. 



They are not a result of the mere accumulation of a series of 

 sedimentary beds ; for, when the last layer of such a series is 

 laid down, the whole is still under water, and some force is re- 

 quired to raise them above the ocean, so as to entitle them to a 

 place among the earth's mountains. And generally there are 

 plications and metamorphism attending upon such elevation, due, 

 directly or indirectly, to the same powerful agency. While, then, 

 they consist mostly of sedimentary beds, altered or unaltered, they 

 have been raised to their places by an adequate force. Mountains 

 lifted by lateral pressure or tension within the crust would be sup- 

 ported as raised ; they would not be resting on a sea of unstable 

 vapors, but would have a solid basis, — that by the movement of 

 which they were elevated. 



6. Epochs of elevation separated by long intervals. — Mountain-chains 

 are not the work of the earlier periods of the globe alone, when, it 

 is believed, the earth's fires were most active, but of particular 

 epochs in the course of all its ages ; and the loftiest of the globe ' 

 received much the larger part of their altitude after the close of 

 the Mesozoic era (p. 503). Plications and disturbances of strata, 

 and metamorphism, have also occurred at intervals in all ages, and 

 the two sets of phenomena were partly cotemporaneous. 



The special epochs of great uplifts and foldings in eastern North 

 America have been shown to be — (1) the later part (or close of the 



