Davis.] 
332 
[Nov. 18, 
to side of the flood plain, always leaving a layer of cobbles at the 
level of its bed, and always building up its flood-plain over the 
cobbles on the side of the channel from which it was swinging 
away. 
The cause of the variation in the width of the flood-plain is 
not clearly apparent. At Walcott’s Mills, M, fig. 3, the flood- 
plain practically disappears, although it is half a mile or more 
in width above and below this point. Here the stream has 
cut through the sands and a little into the bed-rock, which 
rises slightly above the flood-plain level at this locality, and 
for that reason cannot easily swing about laterally. But if 
this be the control of the narrow space here opened it may be 
concluded that the opening of the flood-plain elsewhere was for 
the most part accomplished after the stream had cut down to the 
ledges that hold it at Walcott’s Mills ; and hence that the change 
of level of the land from its former depressed to its present ele- 
vated position was brought about in a small share of the time 
that has elapsed since the period of depression was ended. 
Otherwise, a wider excavation should appear in the sands at 
Walcott’s Mills. But the narrow flood-plain near South Cairo 
does not appear to be controlled by ledges ; at least none were 
visible as I drove up the valley ; nor at Salisbury Manor is the 
river held in a fixed course by the ledge, although the sandstone 
beds then encountered do prevent its easy meandering further 
south than its present course. The control of the width of the 
flood-plain does not seem to be entirely determined by ledges on 
which the channels have been superimposed ; and I have there- 
fore ventured on another supposition, which is here offered 
tentatively. 
At Leeds, shown at the eastern margin of the map, the 
Catskill crosses the Marcellus valley and escapes from it by a gap 
in the Corniferous limestones on the east : it is these hard lime- 
stones, here encountered by the stream for a distance of several 
hundred feet, that constitute the effective- local baselevel with 
reference to which the flood-plain has been formed. At this 
point, where the stream cuts the hard limestones, it can swing 
laterally only at a very slow rate : it is practically held in a fixed 
position, compared with the ease with which it meanders about in 
the valley-filling further up stream. The meanders, however, 
are not altogether at random ; it is well known that far a stream. 
