OSCILLATORY SEAS 511 



The Pacific ocean was dominant during the early Paleozoic, before the 

 'Taconic revolution — that is, during the Cambric and early and late Ordo- 

 vicic, when its waters extended to Appalachia. Subsequently, although 

 often of marked influence, it became dominant only in the Upper De- 

 vonic, the early Mississippi^ early Pennsylvania Permic, Upper Triassic, 

 and late Jurassic. 



It is thus seen that North America was most frequently and persistently 

 inundated by the southern waters of the Pacific ocean and the Gulf of 

 Mexico ; in other words, by equatorial waters. This condition was rendered 

 variable only by the floods from the Arctic ocean, which were decidedly 

 great in the early Paleozoic, but later became spasmodic and much reduced 

 in extent. In transgressions the Atlantic was practically a negligible 

 factor, yet it often made its influence felt through the distribution of its 

 faunas. 



Oscillatory seas. — Suess was the first to point out the fact that strand- 

 lines are oscillatory, but Ulrich has rediscovered this same truth in his 

 studies of the areal distribution of the Ordovicic formations. The term, 

 as here used, is applied to the third order of movements of the strand- 

 lines, namely, those that delimit formations. As employed by Suess, the 

 definition includes both major and minor oscillations, with special empha- 

 sis on the latter. 



It has long been apparent that most formations represent local accumu- 

 lations, yet few persons have distinctly realized the fact that these lenticu- 

 lar deposits are of extraordinary number and are delimited either by 

 variations in the depth of the sea or by short intervals of emergence. 

 Suess states that the oscillations "correspond for the most part with nega- 

 tive phases" of the strand-line, but to the writer they seem to be nearly as 

 characteristic of the positive phases of the strand-line. The American 

 Paleozoic continental seas consisted of very shallow waters. Any move- 

 ment at any time must have affected the strand-line. As the continental 

 seas were distinctly the receiving basins of detrital material, and as the 

 load was very unequal in different areas of the same sea, it follows that each 

 shallow basin must have constantly and locally altered its depth. Further, 

 owing to unequal loading, the subsidence in these areas must have been 

 unequal, with the result that the strand-lines of continental seas con- 

 stantly fluctuated, but were always subject to the major negative and posi- 

 tive movements of the oceanic level. 



Since the continental seas varied so remarkably in depth, extent, charac- 

 ter of deposits, mineral content of waters, showed such constant fluctuation 

 between shallow seas and marsh flats, and were highly sensitive to any 

 ■change in climatic conditions, as that of summer and winter or of warm 



