48 Prof. J. D. Dana on some Results of 



larger part, or all, of the Tertiary era. But the local crushing 

 or plication of these beds cannot account for the elevation ; and 

 no other crushing among the surface-rocks of the mountains 

 can be referred to this era. There may have been a crushing 

 and crumpling of the nether rocks of the mountain. But it 

 must also be admitted that there might have been, under tan- 

 gential pressure, a bending of the strata without crushing, espe- 

 cially if there is beneath the earth's rind along the continental 

 borders a region or layer of " aqueo-igneous fusion," such as 

 Professor LeConte recognizes. 



In the course of the geological history of the North-American 

 continent, there were many oscillations of level in the land. 

 Portions that were raised above the sea-level in one era, in 

 another subsided again and sank beneath it; and Professor 

 LeConte, in the course of his discussion, admits the existence 

 of an elevated region along the Atlantic border which afterward 

 disappeared. Had the elevation in the case of such oscillations 

 been dependent on plication and crushing beneath, so complete 

 a disappearance afterward would have been very improbable. 



Such facts as the above appear to prove that elevatory move- 

 ments have often been, like those of subsidence, among the direct 

 results of lateral pressure. The facts are so well known and 

 the demonstration so generally accepted as complete, that I have 

 suspected that there is here an unintentional omission or over- 

 sight in Professor LeConte' s paper. 



3. Kinds and Structure of Mountains. 



While mountains and mountain-chains all over the world, 

 and low lands also, have undergone uplifts in the course of their 

 long history that are not explained on the idea that all moun- 

 tain-elevating is simply what may come from plication or crush- 

 ing, the component parts of mountain-chains, or those simple 

 mountains or mountain-ranges that are the product of one process 

 of making, may have received, at the time of their original makiny , 

 no elevation beyond that resulting from plication. 



This leads us to a grand distinction in orography hitherto 

 neglected, which is fundamental and of the highest interest in 

 dynamical geology — a distinction between : — 



1. A simple or individual mountain-mass or range, which is 

 the result of one process of making, like an individual in any pro- 

 cess of evolution, and which may be distinguished as a monoge- 

 netic range, being one in genesis ; and 



2. A composite or polygenetic range or chain made up of two 

 or more monogenetic ranges combined. 



The Appalachian chain (the mountain-region along the 

 Atlantic border of North America) is a polygenetic chain ; it 



