AET. 21 OEDOVICIAN TEILOBITES ULRICH 75 



opinion demand elimination of formations from both the top and 

 the base of this system as originally defined by Lapworth. The 

 evidence on which these views are based, particularly those facts that 

 concern the upper boundary of the system, are briefly discussed above 

 and much more fully in the just cited Ordovician-Silurian boundary 

 paper published in 1926. The evidence relating to the lower bound- 

 ary was already rather thoroughly pointed out in my " Revision of 

 the Paleozoic Systems " published in 1911, but some of it will again 

 be presented with local details and in generally amplified form in 

 a work on the Paleozoic formations in Oklahoma due to appear 

 before the close of the present year. 



Genei'alized comments on other colvAiins of table. — Regarding the 

 changes in correlating European and American formations that 

 continued study of the extremely difficult problems in the past three 

 years has indicated, I make no apologies. The innovations are pre- 

 sented as suggestions and not as final conclusions. They are based on 

 theoretical considerations and reasonable inferences and probabilities 

 that are not yet susceptible of satisfactory proof — and may never be. 

 Still, they seem as well worth trying out as other not very dissimilar 

 suggestions were that have been presented in the past twenty years 

 and whose merits have in the meantime been fully established. 



The probable bearing of the postulated differential character and 

 slowness of the vertical movements of the surface of the earth on 

 the correlation of formations in the more or less widely separated 

 geological provinces covered by the table is indicated by the inter- 

 mediate placing of many of the names of the formations in the 

 several columns. However, I am not at all certain that the European 

 and even some of the North American formations actually belong in 

 the positions assigned to them in the chart. Any of these may 

 belong a notch or two higher or lower in the time scale than is indi- 

 cated by the present status and probable trend of the organic and 

 physical evidence studied to date. But I do feel satisfied that the 

 tentative arrangement presented in the chart is a nearer approxima- 

 tion to the facts in the several cases than any previous effort has 

 attained. 



Of extreme and commanding importance in working out the 

 sequence of events and the great length of time involved in the 

 geologic history of the Lower Paleozic ages is the indisputable fact 

 that so far as Icnown the least incomplete depositional record of these 

 ages occurs in America. I venture to say further that, so far as the 

 stratigraphic correlation of the marine deposits of these ages in the 

 several largely supplementing provinces in North America is con- 

 cerned, the record of the frequently shifting Paleozoic epicontinental 

 seas is also better understood than is the more epitomized and on 

 the whole much less completely developed record found m European 



