PHYSICAL AND: FAUNAL EVOLUTION 27 
conformity, and the highest beds are thus wanting. In Center County, 
Pennsylvania, the upper beds appear to be completely represented. 
They are succeeded by 2,335 feet of dolomitic limestones, classed by 
Collie with the Beekmantown, but for reasons given elsewhere’ referred 
by the author to the Chazy; and by 235 feet of limestones of Upper 
Stones River (Upper Chazy) age. ‘The succession seems to be uninter- 
rupted, placing this section in the region of non-emergence, while the 
others cited belong in that of emergence during late Beekmantown 
time. The section in central Pennsylvania does not, however, show 
the base of the Beekmantown, which.is thus thicker than 2,500 feet 
(see Fig. 5). There seems no reason for doubting that the higher 
Mohawk . Central 
Valley Penn. 
- == 
ic 
== 
Hiatus 
Fig. 5.—Diagram showing relationships between the Mokawk and Central 
Pennsylvania sections and the character of the overlaps and “‘off-laps,”’ with the pro- 
gressively decreasing hiatus. 
beds of the Beekmantown were progressively deposited during the 
slow retreat of the sea, and that each higher member had, in general, 
a smaller areal distribution than the preceding one. On this view 
the successive members have the “‘off-lapping” arrangement of 
shingles, except that the earlier and lower formations are continuous 
beneath the higher ones. This is regressive overlap or ‘“‘off-lap,”’ 
and seems to supply the only rational explanation answering to the 
facts. To assume that the whole of the Beekmantown was deposited 
before retreat began, not only makes the negative diastrophic movement 
a cataclysmic one, where the positive movement was a very slow 
t Types of Sedimentary Overlap, p. 6109. 
