310 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



If the "Chemung formation" be extended below the fossiliferous strata of Ithaca, as it 

 was in the literatxu-e before 1880, then the fossils in the "Ithaca group" belong to the Chemung 

 fauna. When the Ithaca fauna was dissected and it was shown that the species were not those 

 of the Chemung famia above but were rather modified successors of the Hamilton fauna, it 

 became clear that, faunally, the Ithaca group was not a part of the Chemung formation. 

 Nevertheless, the term "Chemung" was still retained in general literature for the "period" 

 which included both the "Ithaca" and "Chemung" epochs, so that the real issue was stiU 

 obscured by the imperfection of the nomenclature which used "Chemimg" with two meanings. 



The terms "Portage," "Hamilton," "Trenton," and "Niagara" are also applied in this 

 double sense in the classification of formations, making it almost impossible to frame a state- 

 ment which will express the thought that formations and faunas are discriminated upon 

 different bases and that their limitations may not be identical. 



In order to demonstrate the actual facts in the case, it has been found necessary to collect 

 a large number of statistics regarding the actual faunal contents of each zone in some well- 

 known formation, and also regarding the separate faimules taken from outcrops of the same 

 formation over an extended area. 



It is now possible to state that the Tropidoleptus fauna of the Hamilton formation per- 

 sists in its integrity above the top of the Hamilton formation; that in eastern New York it 

 occupies a place in the column which is occupied in central New York by the Ithaca formation 

 and in the Genesee Valley by a portion of the Portage formation. 



This state of things has been already demonstrated in respect to the position of the CatskiU 

 formation in the geological column. But the significance of the facts was obscured in that case 

 by the fact that the CatskiU as a pure formation is distinguished by its red sedimentation, 

 which, therefore, was easily discerned in the field by the stratigraphical geologist; but the fossil 

 evidence of the Chemung, though constantly annoying him, had not in his mind the distinct 

 stratigraphical significance which he attached to the color ingredient in the CatskiU. The evi- 

 dence of the CatskiU was clear, and if the fossUs told another story, so much the worse for the 

 fossils. This was his attitude. 



In the present case the faunas are of the same kind, made up of marine invertebrate fossils. 

 They are distinctly marin^ in all cases, and the demonstration may be expressed in mathemat- 

 ical values. The statistics are sufficient and are gathered from a field that is wide enough to 

 make possible the comparison of the faunules in terms of composition, frequency, and abun- 

 dance. The variation of species, though not yet demonstrated by the statistics, is strongly 

 indicated by the increasing uncertainty in identification of the species in one direction, while 

 the species are always positively identified in the central region. Great promises of futiu-e 

 discoveries in this direction are offered by the facts, and in the future we may expect to see the 

 laws of variation associated with transgression of the faunas clearly demonstrated. 



Enough evidence is already in sight to show that at any particular point of time, as repre- 

 sented by a common geological horizon or zone in a given formation, the inhabitants of one 

 sea differed in species within a relatively small distance (50 nules) ; and within 200 miles the 

 faunas may be entirely different, having not a single species in common • 



The facts also give clear evidence of the shifting of the fauna with the accumulation of the 

 sediments, so that the center of distribution of each fauna changes as we ascend in the formation. 

 The evidence points to this shifting of the total fauna as the occasion of rapid modification and 

 variation of the species, and the inference is drawn that great changes of conditions were coin- 

 cident with great shiftings of the fauna. During the prevalence of a fauna in a common center 

 of distribution very little evolution took place for long periods of time, as measured by thick- 

 ness of sediments, but slight shifting in the geographical position of the fauna is coincident with 

 the appearance of new varieties and, in general, with disturbance of the faunal equUibrimn. 



The work of dissecting the contents of a fauna into its constituent faunules and then of the 

 analysis of these faunules into their specific composition was begun at Ithaca, in the midst of 



