510 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



with it. Again, but this time referring to a larger unit, the discovery 

 of Rhynchotrema capax, or Rhombotrypa quadrata, or Sceptropora f acuta, 

 would be deemed sufficient evidence on which to base an identification of 

 Eichmondian deposits. It is to be said, however, that while these fossils 

 seem to be diagnostic of the Eichmondian over the whole northern 

 hemisphere, none of the three is very useful in distinguishing the several 

 zones of the series. Other species of more limited geographic range are 

 used for this purpose. 



Doubtless most of the larger divisions of the time scale and probably 

 many of the inferior units are identifiable by equally trustworthy guide 

 fossils. Their recognition and final selection is a matter solely of acquired 

 detailed knowledge and extensive tests in the field. Obviously, their 

 practical value in correlation is greatest when they are striking objects, 

 easily recognized and common enough to be quickly found when needed." 



(5) Sequence of life zones. — The sequence of minor hut well defined 

 life zones, luhen found to agree in loidely separated localities, is to he 

 regarded as highly significant in establishing the essential contempo- 

 raneity of the respective zones- The value of the principle is enhanced 

 by the fact that the identification of each zone checks the correlation 

 of the other zones. However, on account of the oscillating character 

 of continental seas, the application of this principle, except in broad 

 correlations, is necessarily limited to sections within the same province 

 or basin. Its chief source of difficulty arises from the same cause, namely, 

 on account of differential movements (many are described in the first 

 chapter of this part) the sequence of beds and fossil zones is not exactly 

 the same in any two areas of uplift, nor even on opposite sides of the 

 same "dome.'' Bearing these frequent local imperfections of the record 

 in mind, the principle of similarity in sequence is very useful in corrobo- 

 rating and checking correlations of the individual zones. 



A good example is found in comparing the successive changes in the 

 Eden-Maysville fauna at Cincinnati with those in the Utica-Lorraine 

 fauna in New York. The well defined faunal zones in the former sec- 

 tion are more easily recognized in the latter than I believed before making 

 a personal field study of both. Perhaps the best example is that of the 

 Ordovician, Silurian, and Devonian life succession in the Cincinnati and 

 Nashville uplifts. The lithological aspect of this instance is discussed 

 on page 526. 



(The frequently abrupt passage from one to another of these life 

 zones, whether their faunas are new or recurrent, is thought to indicate 

 either a change in the contributing oceanic basin or withdrawal of the 



