378 E. O. ULRTCH — REVISION OF THE PALEOZOIC SYSTEMS 



Cenozoic rocks have remained practically fixed in the past fifty years and 

 more, those of the Paleozoic have been in a state of almost continual modi- 

 fication. The Paleozoic column, though occasionally suffering reversals, has 

 on the whole grown steadily ; and this growth has not been solely through 

 the addition of new stratigraphic units. No, geologists have learned to 

 detect discontinuities in sedimentation where formerly none was sus- 

 pected. In a majority of instances these hiatuses doubtless represent 

 long periods of land emergence. Locally, the physical evidence for this 

 interpretation is often exceedingly obscure. Indeed, considered by itself, 

 the physical evidence of unconformity in many cases may seem quite 

 inconclusive, even when the corroborative evidence of fossils and the 

 criteria of overlap or of retreating seas proves the break to have a great 

 time value. 



That the accessible marine and land deposits nowhere comprise a 

 continuous record of geological history none will deny. The principle is 

 so firmly rooted and has been for so many years, that it would be a waste, 

 of time and opportunity to argue the point. Yet, despite the general 

 acceptance of the principle, much difference of opinion prevails con- 

 cerning the aggregate and individual extent of the imperfections. Some 

 probably would maintain that the imperfections are largely due to in- 

 complete observation and study of the stratigraphic record, that only a 

 small part of the continental marine sediments of the earth has been 

 carefully investigated, and that when our observations have been extended 

 so as to embrace all the accessible deposits the imperfections will have 

 become practically negligible. So far as the biologic record is concerned, 

 this optomistic view is probably justified, since with time the unflagging 

 efforts of investigators doubtless will fill most of the gaps occasioned by 

 regional variations in biologic development; and, finally, the time gaps 

 will also have been largely accounted for by refined methods and practice 

 of stratigraphic correlation. But past experience suggests the futility of 

 the hope of bridging all the gaps, or even of fully appreciating their 

 significance. 



With the advance of knowledge the gaps are lessened by the inter- 

 calation of previously unrecognized fragments of the record, but the gaps 

 themselves, however minimized, are yet multiplied. It is commonly 

 found that the intercalated beds still fail to completely fill the gap, so 

 that instead of the previously known hiatus two or more smaller gaps 

 remain to be accounted for. 



The true time significance of a stratigraphic hiatus is never appreciated 

 until beds corresponding to it in whole or part are discovered. The 

 faunas, in the absence of information respecting the unknown stages of 



