634 ELIOT BLACKW ELDER 



which base-leveling of the surface is widely extended.^ Each 

 period of quiescence and stress-accumulation is terminated by a 

 short epoch of deformation during which the rocks yield to the 

 overpowering stresses to such an extent as to produce temporary 

 equilibrium among the stresses and thus inaugurate a new period 

 of quiescence during which the cycle of planation of the surface 

 may be repeated. Illustrations of the principle may be found in 

 the history of any of the well-known mountain systems such as the 

 Appalachians, the Rocky Mountains, or the Alps and their con- 

 temporaries. 



For the purposes of the present paper, the sharp foldings 

 and often intense plications of strata are to be discriminated 

 from those gentler but more widespread movements generally 

 termed "warping." With the latter are often associated faulting 

 of the so-called normal type, and volcanic activity. These things, 

 although different in kind, are not unrelated. There is ample 

 evidence that rock folding is often accompanied or followed by 

 volcanic action; and one of the phenomena attending the plication 

 along a narrow strip may be much more widespread warping with 

 more or less normal faulting. Nevertheless, it is not clear that all 

 four are necessarily present as phenomena of any one period of 

 readjustment. In the strict etymological sense, folding, warping, 

 faulting, and volcanic action are all "oro-genic" disturbances 

 (oro5 = " mountain," gennao = '' to produce"), since each may, 

 independently of the others, give rise to mountain forms. In this 

 paper, however, the word "orogenic" will be used in a somewhat 

 limited sense and applied only to those epochs characterized by 

 prominent rock folding.- 



Recent textbooks and other general works on the geology of 

 North America display an interesting lack of agreement as to the 

 number of orogenic epochs recognized, in the emphasis placed upon 



^ As an additional but quite distinct factor in lowering the lands Chamberlin also 

 introduces the idea of a slow glacier-like creep of the continental masses outward toward 

 the ocean basins during the quiescent periods. Since planation and crumpling may 

 alternate, whether body creep is a real process or otherwise, such a process will not be 

 considered in this paper. 



^ The term "revolution" is often given to these disturbances but appears to be 

 too strong a word, in view of the frequent recurrence of such events, the world over. 



