292 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



has often been disconcerted when the unquestionable succession of beds 

 showed he had gotten things upside down or on the wrong plane. And 

 he knows better than any one else that his information on the mutation 

 and succession of faunas and floras is far from complete. Still it is a 

 fact that fossils in experienced hands afford by far the most competent 

 and reliable evidence now available in stratigraphic correlation. Like- 

 wise in the elucidation of the physical as well as the organic history of 

 the earth fossils play a most important part. Finally, it is true that 

 without their aid stratigraphy could never have become a science. The 

 biologic part of paleontology may be considered separately from geologic 

 processes, but stratigraphy can not exist apart from paleontology. 



Investigations tending toward Eevision of stratigraphic 

 Classification 



the natural basis of geologic time divisions 



Nearly twenty years ago T began to entertain doubts as to the prevailing 

 classification of Paleozoic rocks. That crustal movements considered in 

 connection with faunal evidence afforded the best means of revising the 

 classification on a sound and consistent basis did not appeal to me till a 

 few years later, when I was engaged on the correlation of the Ordovician 

 beds of Minnesota.^ The idea was too new and vague to be employed 

 then. It had first to be tried out in the field. Since my connection with 

 the Federal Survey in 1897, I have enjoyed abundant opportunities to 

 apply and test its principles. In my opinion, the results are eminently 

 satisfactory. 



The theories advanced in ^^Seas and barriers in eastern ^N'orth Amei- 

 ica'^ by Ulrich and Schuchert, in 1902, have been sustained in every 

 essential feature by subsequent investigations.^ Chief among these is 

 the permanence of certain continental lines of elevation separating areas 

 of depression. Along tbe former, discontinuities in sedimentation and 

 subsequent overlaps recur frequently, while in the intermediate spaces 

 deposition was much more continuous. Obviously the areas of frequent 

 elevation and consequent emergence afforded the best, and sometimes the 

 only, evidence of crustal movements. Assuming the general truth of 



2 Geological and Natural History Survey of Minnesota, Pinal Repts., vol. iii, introduc- 

 tion, 1896. 



8 The only important modification of the views expressed In the work cited is that the 

 deposits on opposite sides of the Appalachian barriers are as a rule not contemporaneous 

 but successive. In other words, that the concerned continental seas were smaller and 

 shifted from time to time, oscillation causing repeated sea withdrawal from troughs to 

 the east of the barriers before other waters filled the basins to the west of them. This 

 modification removes the principal objection to the original theory, in that it eliminates 

 the diflBcult conception of long and very narrow land barriers. 



