284 The American Geologist. ^^>'' i^'^^. 
the creek (which must have been the agent in producing the 
change) must either have cut down a hanging adjustment or 
nuist have cut, at its mouth, a new rock-bound channel down 
to the level of the present adjustiuent. This achievement is 
discussed in No. 20. 
19. It is quite difficult to see how the age of the original 
valley, and of the upland mantles can be excluded from anv 
geological investigation of this problem, unless, by an arbitrary 
and quite precipitate assumption, the deposit in (iuestion 
is entirely a modern accident, and all the surroundings must 
be made to agree with that accident as a starting-point. But, 
allowing the exclusion of these questions except on the speci- 
fied condition, it appears to the writer that even on that con- 
dition the age of the original valley and of the upland mantles 
is very essential to the investigation, for that specified condi- 
tion as shown in No. 6 is plainly supplied. 
20. In. other words: it is possible that a tributary creek, 
rising say 300 feet above the Missouri, running in a valley that 
is cut in the Carboniferous rocks at least 200 feet, down to the 
level of the Missouri at Concannon's and at least 24 feet be- 
low its present floodplain, may be in its original gorge except 
near or at its mouth, where it may be in a new channel fash- 
ioned by recent agencies. If that is possible, it is possible for 
a creek to get out of its rock-bound gorge at some point 
above its mouth, and at its mouth to'cut a new gorge at least 
100 feet in depth by the operation of "recent agencies" — i.e. 
since Wisconsin glaciation — a task which, so far as known to 
the writer, no creek has ever accomplished, especially no "per- 
iodic run off." The valley at its mouth is rock-bound on 
both sides. If professor Chamberlin's supposition does not 
refer to this rock-cut channel, but to the present shifting posi- 
tion of the little dry channel that meanders through the creek's 
floodplain at its mouth, it is necessary to admit that recent 
agencies have determined its position. But that contravenes 
his hypothesis, since the creek's antecedent valley, as already 
shown once occupied by the creek, continues to the Missouri 
at the same place underneath the present creek, and the pres- 
ent floodplain and its little dry channel are but accidents de- 
pendent on the vicissitudes of everv season. If bis hvpothesis 
refers to any other stage in the waters of the creek, between its 
