452 E. O. ULRICH— CORRELATION OP THE STRAND-LTNE 



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Fauna of the Mendota dolomite, au early Ozarkian formation 477 



A Helderbergian invasion of the Onondaga coral fauna 478 



Faunas of distinct marine provinces differ in composition and se- 

 quence 4S0 



Examples showing tentative nature of correlations based on sup- 

 posedly characteristic genera 481 



Expansion of the vertical range of Gonioceras 482 



Age determination solely by percentage of species known elsewhere . . 484 

 Proper use of fossils in correlation 488 



Introduction 



EFFECT OF VARYING CIRCUMSTANCES ON PRINCIPLES AND CRITERIA OF 



CORRELATION 



The practice of stratigraphic correlation — that is, the determination of 

 the time relations of marine and continental deposits in widely separated 

 areas by other means than continuous tracing of beds — is rarely a simple 

 process. Often, indeed, the problems to be solved prove exceedingly in- 

 tricate. With the progress of the science many rules and criteria for 

 establishing contemporaneity in geologic events have been suggested and 

 applied with varying success in the practical work of the geologist. At 

 first most of these rules and criteria seemed sound in principle and 

 widely, if not universally, applicable ; but as our stock of facts increased, 

 the correlations indicated by them became more and more uncertain. 



At present there are no formally recognized rules that, taken either 

 singly or in combinations, will insure uniformly reliable results under all 

 circumstances. Criteria whose Avorth has been proved for one geological 

 province fail more or less decidedly in another, and their failure is not 

 in proportion to the distance between such provinces, for it may be greater 

 where the separation is measured by only a few miles than in other in- 

 stances in which provinces of different continents are involved; in fact, 

 the mere matter of distance plays but a subordinate part in the contem- 

 porary variability of faunas and floras. Fossil marine faunas, for in- 

 stance, may extend in essential purity for great distances in one direction, 

 whereas in the opposite direction they may cease abruptly and perhaps 

 appear to give way to a totally different fauna. 



Such abrupt changes in the fossil contents of beds similar in lithologic 

 character and apparently occupying corresponding positions in the geo- 

 logic column may seem difficult to explain ; and instances of this kind are 

 not at all uncommon in the Appalachian region. However, without ex- 

 ception the faunally distinct, though apparently contemporaneous, beds 

 have proved to be not only of different ages, but, what is of much greater 



