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E. O. ULRICH REVISION OP THE PALEOZOIC SYSTEMS 



physical conditions similar in expression and effects to those prevailing 

 on the earth of today. 



As must be plain from preceding statements, indeed from the whole 

 tenor of the present work, I can not subscribe to these views. In the first 

 place, tlie continental seas were but seldom large and never deep, nor did 

 they endure through many geological ages. On the contrary, they were 

 limited in extent and subject to frequent oscillations and withdrawals. 

 Next, the geographic changes in nearly contemporaneous fossil faunas 

 are due less to local physical conditions than to original peculiarities of 

 the faunas of the oceanic basins from which the different continental 

 seas happened at the time to draw their organic supplies. Finally, 1 find 

 no warrant for the assumption that great sheets of marine deposits have 





A I 



Figure 2. — Illustrations of marginal Erosion 



The sketches iUustrate marginal erosion of deposits in a warped or slightly folded 

 basin, with subsequent deposition of a conglomerate partly composed of material derived 

 from the exposed edges of the lower beds. Bed 2 should haye been represented as reach- 

 ing the surface at the left of the lower sketch. 



been entirely eroded away. It seems rather that nearly all surface degra- 

 dation of considerable magnitude has ever been confi.ned to the positive 

 areas. This commonly involved the edges of the marine sediments, which 

 overlapped the flanks of such upwarped areas, and sometimes also the 

 thinning sheets covering them were removed. But over the broader nega- 

 tive areas, particularly in the Paleozoic, erosion processes have always 

 been comparatively negligible in their effect on consolidated deposits. 



It is not my intention to deny that great elevations have been reduced 

 at times to baselevel. Indeed, as is indicated by occurrence of clastic 

 deposits in certain sections, I think this occurred repeatedly in the same 



