correlating Geological Formations. 431 



are so variable that the form Z. matheri is not conclusive of 

 post-Ithaca stage. 



In my collections from the Waverly-Chemung cliffs, how- 

 ever, Tropidoleptus was discovered above the first appearance 

 of Spirifer disjunctas and other typical members of the Spirifer 

 disjunctus fauna. These facts are intrinsic evidence, therefore, 

 that the combination of speeies, so much like the typical Tro- 

 pidoleptus carinatus fauna of the Hamilton, is here present in a 

 part of the rock section occupied in general by a typical Spi- 

 rifer disjunctus fauna. 



The fact that the combination of species is the normal com- 

 bination seen in the undisputed Hamilton formation shows that 

 its equilibrium has not been disturbed and, therefore, that the 

 life history of the fauna of the Hamilton formation has not 

 ceased ; while the faunas above and below in the cliffs in 

 Chemung narrows is evidence that the geological horizon is 

 that of the typical Chemung formation. The lapping of faunas 

 of the same kind seems to be established by evidence beyond 

 dispute, and correlations must be made with recognition of 

 such a possibility in cases where the direct evidence of the fact 

 may be wanting. 



When we attempt to correlate formations with this knowl- 

 edge before us, it is evident that the life period of a fauna is 

 not what it appears to be in any particular section. Whenever 

 the succession is sharply defined by the stopping of one fauna 

 and the abrupt beginning of another, in full or decided 

 strength, the evidence should be interpreted as positive that 

 the boundary between the two consecutive formations does not 

 make the end of one fauna and the beginning of the succeed- 

 ing one. It is to be interpreted rather as only a well advanced 

 stage into the later one, and the vigorous period of persistence 

 of the other. This, interpreted into comparative terms, would 

 result in showing that the two faunas lap over each other in time. 



My studies convince me that this is frequently the case in 

 respect to the boundary lines of our formations. The abrupt 

 transition from one formation to another with a different 

 fauna is convincing evidence that the abruptness of the change 

 in fossils is due either to absence of strata (i. e., an apparent or 

 concealed unconformity) or else to migration of the faunas 

 across the area. 



This principle must be recognized in making correlation, if 

 we would reach correct interpretation of the facts. 



The laws discovered in the statistics above reported may be 

 summarized as follows: 



A geological fauna may be defined as an aggregation of 

 species living together, the several species of which hold a 

 definite value in relation to each other. 



