402 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



events, is the level of the sea. Even this statement must be qualified, 

 since, under the perhaps well grounded conception of a periodic equa- 

 torial heaping of the waters, the level of the several oceans may rise or 

 fall in the same measure only at the same latitude, and then only when 

 the proportionate bulk and altitude of adjacent land-masses remains 

 unchanged. But, whatever the qualifications, there yet remains the fact 

 that the strandline is contemporaneously and universally displaced. Ob- 

 viously the exact correlation of these displacements at times of great and 

 diversely manifested diastrophism, as at the close of periods and eras, 

 is more difficult than are the more eustatic migrations of the strandline 

 which occurred at intervals within the durance of the periods. Doubt- 

 less the times of excessive crustal deformation are everywhere indicated 

 by some marked change or break in sedimentation ; but the break is likely 

 to be a long one and the determination of its time relations in two or 

 more areas correspondingly indefinite. During parts of the interval rep- 

 resented by a well defined intersystemic hiatus deposition may have been 

 going on elsewhere; and it is sometimes very difficult to decide whether 

 these intervening deposits represent later stages of the old period or earlier 

 beds of the new or, as often happens, of both. In the last event several 

 perhaps equally distinct boundaries may occur within the intervening 

 mass, and as all the criteria relied on in distinguishing the two systems 

 in the first case are more or less weakened through transition, it may 

 indeed become a matter of no small difficulty to decide which of the 

 boundaries is the most important. The Siluro-Devonian boundary in 

 southeastern North America and in central Europe affords a good illus- 

 tration. 



The displacements of lesser rank, such as define series and groups, 

 are more readily amenable to intercontinental correlation. It is not 

 that the deposits of a given epoch can be more easily recognized in the 

 several continents than those of the much larger period unit, but that 

 the av^erage time value of the breaks is less and the competent accessible 

 data more numerous, thus enabling us to reduce the limit of error in 

 determining the equivalence of intrasystemic boundaries to less formida- 

 ble dimensions than when we seek to correlate the terminal deposits of a 

 system. For instance, it will be less difficult to establish the exact time 

 relations of the subdivisions of our Niagaran series to the Silurian zones 

 recognized in England and Sweden by diastrophic criteria — using that 

 term in its broadest sense — than it will be to prove that the Manlius of 

 ISTew York is not represented in the upper part of the Wenlock, or that 

 the Ludlow group — yes, even the Dudley limestone of England — does 

 not include beds of the age of the Helderbergian in America. 



