Time-table for North America. 13 



The long-enduring middle portion of the periods is marked 

 by relative crustal stability, as shown by more or less reduction 

 of the continents to sea-level (peneplanation). The sediment 

 from the eroded lands adds that much volume to the sea and 

 causes its level to rise as a result of the partial filling. This 

 effect, accompanying the general leveling of the land surface, 

 produces, in the middle of the periods, maximum sea invasion. 

 On the other hand, the earlier times of each cycle exhibit less 

 crustal constancy and more marked erosion. The lands then 

 warp more or less along predetermined lines, due to internal 

 adjustments following the major movement and the reestablish- 

 ment of the balance between the sinking and rising areas 

 (isostatic balance or isostasy), which has been altered by these 

 deformations, by the sea invasion, and by the unloading of 

 emergent land areas into the seas of the continents. During 

 the closing stages of the periods, there is a renewal of crustal 

 unrest, seen in the vanishing of the continental seas, which 

 finally ends in another major crustal movement and in more 

 or less complete withdrawal of the seas from the lands. 



There is a certain amount of rhythm in these periodic move- 

 ments and this meter permits us to group the formations into 

 systems or periods. The more active introductory orogenic 

 movements are of comparatively short duration. In contrast, 

 the quieter but broader deformations within the period, of 

 epeirogenic nature, as shown by world-wide movements of the 

 strand-lines (eustatic movements), are of long continuance. 

 Each submergence with the following emergence is seemingly 

 the natural basis for the delimiting of a period. Among these 

 periodic movements some are far more intense and of greater 

 geographic extent than others, at times when mountain ranges 

 in more than one continent are simultaneously or successively 

 in motion. These are the diastrophic grand cycles, or, accord- 

 ing to Dana and LeConte, the "critical periods" or "revolu- 

 tions" in the history of the earth, and they bind, as it were, 

 the chapters into the book of geologic time. 



Chamberlin (1898) has well said that "the ulterior basis 

 of classification and nomenclature must be dependent on the 

 existence or absence of natural divisions resulting from simul- 

 taneous phases of action of world-wide extent. . . Great earth 

 movements affect all quarters of the globe " because " in a 

 globe, all of whose parts owe their positions to the stress and 

 tension of other parts, every rearrangement that rises in 

 magnitude above the limits of local support: extends its influence 

 to the whole." The movements are not heterogeneous, but 

 are periodic, for " the oceanic basins became progressively 

 deeper and more capacious, while the continents became higher 

 (degradation aside). In this assumption . . . there lies, if it be 



