410 E. O. ULRTCH REVISION OF THE PALEOZOIC SYSTEMS 



A reasonable deduction from these considerations is that, with the 

 exception of the noted invasion by polar seas, all submergences are 

 directly ascribable to movements of the second group, and that all nega- 

 tive displacements of the strandline, excepting again the retreat of polar 

 waters, that are not purely local in development, are relatively impulsive 

 and therefore due to or connected with earth movements of the first 

 group. 



Discrimination of general earth movements. — The details of the most 

 active movements which resulted in great land growth are often very 

 difficult to decipher. In these cases the competent sedimentary and 

 organic criteria are, if not wholly, at least mostly, confined to parts of 

 interior basins and to extracontinental areas now inaccessibly buried. 

 We have, therefore, no adequate means of discriminating between the 

 grander and the less important movements that took place during a 

 highly emergent phase of the continents. Indeed, the land record is as 

 a rule drawn in broad and none too definite lines. When it indicates 

 diastrophic movements, the record shows both the major and the minor 

 movements in a composite picture in which the individual occurrences 

 can not be satisfactorily discriminated. Moreover, since the movements 

 exerted themselves chiefly in areas and lands offering the least resistance, 

 and as these non-resistant areas grew continually weaker and were thus 

 perpetuated, it has become extremely difficult to say how much of the 

 composite effect really belongs to the period to which it is commonly 

 assigned and how much to preceding and succeeding periods. Appa- 

 lachian folding certainly was not all accomplished at one time or during 

 a single period. It began in a pre-Cambrian era and probably is going 

 on in the present (see pages 43G to 439). 



The grandest of the movements referred to in the preceding paragraph 

 are thought to have occurred about the close of the eras. As they left 

 deeply impressed records and their periods were long, the times of their 

 occurrences are recognized the world over. Other movements of major 

 import are indicated in the intersystemic intervals. These also are re- 

 corded in a recognizable manner — chiefly by evidence of great sea with- 

 drawal — on all of the continents. Then there were movements, defin- 

 ing series, groups, and the more important formations, that resulted in 

 similar though not always general evacuations of the marine continental 

 basins. These, too, have left intercontinental physical records that will 

 be correlated more or less definitely when the known facts are revised 

 and mere matching of fossils shall have given way to comprehensive 

 modern methods of determining stratigraphic equivalence. It will be 



