736 ARTHUR C. TROWBRIDGE 
of softer material from above, provided the streams were not able 
to cut through the hard formation for a long period of time. Such 
a plain would be essentially a peneplain, with the difference that 
its position is determined by a hard stratum, instead of by base 
level, and that it would require no uplift to start its dissection; 
dissection would result when the main stream had cut through the 
hard formation. It would have essentially the same character- 
istics as a true peneplain, with the marked exception that it would 
be parallel to the layers of rock, and remain at the same strati- 
graphic position over wide areas. f 
If the Niagara plain be tested by these criteria, again no definite 
conclusion can be drawn. As the flat is everywhere located on 
the Niagara, lies everywhere near the top of the massive member 
of that formation, and about 150 feet above its base, and as it has 
all the other characteristics of a true peneplain, it may be said to 
fit the second hypothesis better than the first. 
3. Or is the Niagara plain the original surface which emerged 
from the sea at the end of the Niagaran epoch, and has suffered 
dissection since then? A surface having had such a history should 
have somewhat different characteristics from one developed in 
either of the ways previously outlined. (a) There would be no 
marked hills standing above the remnants of the plain, for there 
were no other strata above it from which hills could have been 
formed. (6) The thickness of the underlying formation would be 
expected to be uniform, except for possible differences in the amount 
of deposition on the bottom of the sea, or erosion since emergence 
from the sea. (c) The plain would have little or no relief, unless 
roughened after its emergence, by streams, wind, etc. (d) Like 
the plain developed on a hard layer, the surface would follow the 
rock structure, not rising or falling appreciably with reference to the 
strata. (e) Any slope the plain has would be the slope, in direction 
and amount, of the old sea bottom, unless tilting occurred during 
emergence or later. (/) No river detritus should be found on this 
surface, but instead material deposited at the shoreline as the shore 
receded over it, if anything. (g) In this case also, if the bottom 
of the old sea has been preserved as the plain, it might be expected 
that somewhere the old shorelines might be found as raised cliffs, 
