576 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



regions where existence was still possible. In the shallow continental seas 

 life conditions could never have continued unbrokenly favorable through 

 long ages ; hence the faunas which inhabited them required continual re- 

 plenishment. And it is to be remembered that geological time was always 

 long enough for the consummation of every such purpose. Also that the 

 fossil record of these small breaks in the life history of the beds is rarely 

 measurable stratigraphically. 



The modification of species and faunas that originated and developed 

 in the same oceanic basin was very slow. This is shown by comparison of 

 successive invasions from a given oceanic basin. Even when we can prove 

 by stratigraphic evidence that long ages intervened the change is often 

 deceptively small. The Hamilton, the Spergen, and the Catheys-Fair- 

 view "recurrences'' discussed in foregoing parts of this work are convinc- 

 ing illustrations. And there are many other instances of confusing simi- 

 larity in general aspect of long separated faunal invasions. Compare, for 

 example, the late Stones Eiver (Lebanon and Carters) fauna with the 

 Lowville fauna. These two appearances of the Gulf of Mexico fauna are 

 in general very much alike, and except for the constant presence in each 

 of a few well marked diagnostic species their discrimination would be a 

 matter of grave difficulty. And yet, as shown on pages 554 to 557, the 

 Lowville is sufficiently younger than the last of the Stones River to per- 

 mit deposition of the Blount group, aggregating between 3,000 and 4,000 

 feet of limestone, shale, and sandstone in east Tennessee. 



When a decided generic change is noted between two immediately 

 superposed faunas it means one of three things. Often it signifies that a 

 great stratigraphic hiatus lies between them. In other cases the two 

 faunas invaded from different oceanic realms. In the remaining cases 

 some diastrophic change occurred within an oceanic realm, in different 

 parts of which two or more facies may have been developing simultane- 

 ously. In the first two conditions the causes are obviously sufficient to 

 produce the noted effects. The last cases, however, are not so simple. In 

 these we have no sure means of determining what actually took place. It 

 may have been an extraordinary restriction of normal habitat, as when 

 great emergence of the continental shelf occurred. Or it may be that 

 some land barrier, perhaps an intercontinental connection, was sub- 

 merged, permitting influx of types previously excluded. Again, a passage, 

 as, for instance, that between the Caribbean Sea and the Gulf of Mexico, 

 may have been closed at times when the Antilles were connected with the 

 Floridian peninsula. Finally, an intercontinental land connection may 

 have been temporarily established, as between eastern Canada and north- 

 ern Europe, which would have favored shore migration that at other times 



