738 ARTHUR C. TROW BRIDGE 
be found on the plain, either cemented into hard rock or 
uncemented. | 
Again the Niagara plain has some of these characters, but not 
all. So far as slope and the apparent original relief and the uni- 
formity of thickness of the underlying formation are concerned, 
the plain might be explained along the lines of this hypothesis, 
but the correspondence of the surface with the structure of the beds, 
the absence of near-shore marine deposits, the absence of all shore- 
lines, and the fact that no other large plains are known which seem 
to have been made in this way, would seem to point to some one of 
the other hypotheses as the true explanation of the flat. 
Conclusion.—After this discussion it is apparent that the origin 
of the Niagara plain is from this region not perfectly clear, but that 
some speculation at least is warranted. The facts that the plain 
is so distinctly related to structure and that it shows no conclusive 
evidence of having been peneplained point toward the conclusion 
that it is the original plain or a structural plain (or both), rather 
than an old.peneplain or a plain of marine erosion. It is unlikely 
that a peneplain would happen to be formed so closely parallel to 
structure as this flat is over wide areas. Stillit is possible that, when 
traced into other regions, this parallelism will fail, and the flat 
will be proven to be a peneplain. It has been so long since the 
Niagaran epoch, that it appears extremely unlikely that a flat 
emerging from the sea at that time could have remained unde- 
stroyed until now, unless perhaps it be conceived that the surface 
took a position very near to sea-level from the start, and held that 
position until uplifted in relatively recent times. Perhaps it is a 
combination of a true peneplain and a structural plain, the flat 
having been developed on the hard Niagara dolomite at a level 
which was near grade for the streams of the region at that time. 
Possibly the flat is an original marine plain and also a structural 
plain, the last deposit in the sea having been made into hard rock 
(the massive Niagara). Most likely the plain under discussion is 
an ordinary structural plain, the surface having been developed due 
entirely to the hard Niagaran dolomite which held up the streams 
at that level until all overlying strata were removed, before the 
dolomite formation was finally cut through and dissection of the 
plain began. 
