448 E. 0. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



into a landward one resulting in slight continental doming. It is con- 

 ceivable that between these and perhaps other opposing stresses the 

 median continental areas pulsated back and forth, the balance of move- 

 ment favoring continental creep, first in increasing, then in decreasing 

 ratio, until such time when sufficient oceanic stress had accumulated to 

 reverse the balance of movement and thus to inaugurate a new cycle. 

 Such an ideally rhythmic progress of geologic events is, of course, 

 impossible in an earth composed, like ours, of varying elements. But, 

 granting considerable modification by more or less adventitious cir- 

 cumstances, which indeed in the course of geologic ages have become 

 more and more diverse and important, I yet believe that diastrophic 

 movements were governed by a plan not greatly different from the one 

 here briefly outlined. Judged by the diastrophic history of Paleozoic 

 periods in America, it seems a reasonable working hypothesis. Each 

 of these periods, so far as worked out, indicates a preceding, sometimes 

 very long, stage of general emergence, during which baseleveling agencies 

 were active, followed presently by a long stage of oscillating sea advance 

 and then by a shorter stage of oscillating sea withdrawal which finally 

 passed into the next general emergent stage. (See also chapter on dis- 

 placements of the strandline.) 



Structueal Criteria 



general disgus8i0n 



The structural phenomena which have a more or less direct and corre- 

 spondingly exact bearing on stratigraphic correlation and taxonomy are 

 those indicating either positive or negative displacements of the strand- 

 line. Positive displacements — that is, advance of shorelines — are indi- 

 cated (1) by progressive overlap of sediments,*^ and (2) by peculiarities 

 in kind and arrangement of the initial deposits. Negative displacements 

 or sea withdrawals are indicated (1) by evidence of shallowing, (2) by 

 imperfections of the sedimentary record, beds or formations of ages 

 elsewhere represented by deposits being absent, (3) by obviously uncon- 

 formable relations of adjoining beds, and (4) by less clearly manifested 

 evidence of discontinued marine sedimentation and emerged or land 

 conditions. The general aspects of these phenomena, also many exam- 

 ples, are more or less fully discussed in preceding parts of this work. 



*9 As used In this work the term "progressive overlap" refers only to the progressively 

 spreading deposits of an advancing sea. "Regressive overlap," on the other hand, is 

 applied only to the areally diminishing deposits left by a retreating sea. 



