correlating Geological Formations. 421 



formations ; and the facts are at hand to show that the actual 

 limits of a fauna, when followed laterally with great care, so 

 as not to lose track of the stratigraphic continuity, transgress 

 the limit and run higher up as we cross the outcrops ; and that 

 it may be said with truth, that marine faunas of the same gen- 

 eral nature, but of different place in the succession in any par- 

 ticular section, may lap over each other stratigraphically. These 

 facts I have been able to demonstrate in respect to the relations 

 of the faunas of the Devonian, which I and a number of 

 geologists who have been trained to use the necessary precision 

 in observing and reading the fossils, have brought to light. 

 The result reached has been this, viz : the evidence is conclu- 

 sive that the fauna of the Hamilton formation, which will be 

 called the Tropidoleptus fauna, prevails in the rocks in 

 eastern New York upward, beyond the place of the Genesee 

 shales, to and beyond the interval occupied by the Ithaca 

 formation of the Cayuga Lake section, and is seen in a few 

 cases of definite, pure successional faunules up to the part of 

 the section occupied by the Chemung formation, whose fauna 

 may be called the Spirifer disjunctus fauna. This same inter- 

 val, stratigraphically considered, is occupied by the Portage 

 formation of the Genesee River Valley section, in which no 

 trace of the Hamilton species and only slight trace of the 

 species of the Ithaca formation are present, as determined by a 

 careful search of the rocks from bottom to top. 



In the elaboration of these facts it was necessary to form a 

 nomenclature with which to handle the several different groups 

 of facts for their discussion in relation to each other, to the 

 element of time, and to the position in the structure scale. 



Fauna and flora are terms well understood where only one 

 point of time is considered ; but where, in addition to the geo- 

 graphical distribution of species, we have also to deal with the 

 geological range, it becomes essential to consider the fauna 

 more exactly. 



Geologically the element of evolution comes in, and a geo- 

 logical fauna is not only a set of species situated in or inhabit- 

 ing a restricted geographical area ; but geologically a fauna 

 must be conceived of as changing its position, and as deter- 

 mined by the preservation of the equilibrium of the several 

 species constituting it. 



This equilibrium is expressed in terms of relative numbers 

 and strength of the species of the fauna. In every aggregate 

 of living species, the individuals of one species are abundant 

 and vigorous in growth ; others are less so but common ; 

 others still are rare and inconspicuous, while an occasional 

 species is represented only by a very occasional specimen. 



