44 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 211 
This state of things has been already partially demonstrated in 
respect to the position of the Catskill formation in the geological 
column. a But the significance of the facts was obscured in that case 
by the fact that the Catskill 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 Catskill. The evidence of the Catskill was clear, 
and if the fossils 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. The} r are distinctly marine in all cases, 
and the demonstration may be expressed in mathematical 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 abundance. The variation of species, 
though not yet demonstrated hy 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 future 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 represented 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 miles); 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 modi- 
fication and variation of the species, and the inference is drawn that 
great changes of conditions were coincident 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 thickness 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 equi- 
librium. 
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 the abundant 
«Dual nomenclature in geological classification: Jour. Geol., Vol. II, 1894, pp. 145-160. 
