Williams.] SHIFTING OF FAUNAS. Ill 
have layers of clearly defined conglomerate. They rarely contain 
any purely marine life, except lingulas. The organisms they do 
contain are generally fish and a few large lamellibranchs (Amnigenia) 
which possibly were fresh- water mollusks, and may have occupied a 
place similar to the unios of the present time. Plant remains of 
unmistakable land origin are frequently found in the sediments. 
Thus in this fourth class of sediments the indications of nearness 
of shore are very clear, not only in the nature of the sediments them- 
selves, but in the organic remains buried in them. Bearing in mind 
this fourfold classification of the sediments, geographically arranged, 
it may be assumed that the relationship they bear to. each other is in 
general coincident with distance from a shore outside of which they 
were laid down. The fourth represents the deposits nearest the shore; 
the third the zone of littoral sediments, rich in organic marine life. 
Going still farther outward from the shore line the more or less bar- 
ren sedimentation is found beyond the zone of the littoral fossils, but 
still near enough to the surface to be influenced by wave action and 
by local and temporary disturbance of the currents and supply of 
sediments; still beyond this are the sediments of the first class, above 
enumerated, which are beyond the reach of movements of currents, 
or oscillation of supply and distribution of the sediments derived 
from the shore. 
We have here, then, a set of formations which are associated with 
different faunal populations, and, although they may be supposed to 
be synchronously deposited, the several formations, discriminated for 
particular regions where each one is typically expressed, possess 
almost nothing in common. The stratigraphical, the lithological, and 
the paleontological characters are distinct for each one of the four 
classes of formations. 
The relation which these four classes of sediments bear to one 
another, and the way in which they stand related in the stratigraphical 
succession of a single section, lead one to the hypothesis that they 
represent approximately relative distances from the original shore line. 
With this as a working hypothesis, it is evident that a shifting 
which might be observed in one of the zones of sedimentation should 
be recognized by a corresponding shifting, in the same direction, of 
the other zones of sediments. 
When it is observed that the Tropidoleptus fauna slops in the sec- 
tions of western New York with the deposit of the Genesee shale, 
while in eastern New York the dominant species of the fauna con- 
tinue on for several hundred feet of strata above the horizon of the 
Genesee shale, the inference is justified that not only has the Tropi- 
doleptus fauna shifted eastward, but that the Genesee shale of the 
western New York section shifted eastward to cut it off, and that a 
place may be evident in the eastern extension of the Genesee sedi- 
mentation corresponding to the Portage phase of sedimentation. 
