IV  .»~ 4*" t §j . . » ·   ' V   "*·_”  r·
  y ze ras canrrr. j 
  ‘L we might do this if we did not take the time to make the simple pr0— ‘ 
1 . visional calculation. ~` 
We will next consider some other cases where we are in danger of    »· .
.   making the opposite error :  
  The Cumberland river, a wide, shallow and somewhat gently-flow  
  _ tng stream in its upper course—from where it hreaks through the  
E , Pine mountains at Pineville to about 25 miles below Williamsburg,  
» in all about 80 miles-—here suddenly narrows up and plunges into  
Cumberland Falls, 05 feet over a precipitous escarpment of the car-  
boniferous conglomerate, into a wild, narrow gorge, trenched some  
—· four or live hundred feet into this tame conglomerate formation.  
Through this narrow canon, filled at the bottom with chaotic accum·  
ulation of large bowlders, the river finds its turbulent way by a suc-  
» cession of pools, rapids and cascades. This continues for a distance  
of some six or seven miles, before the cliffs recede somewhat and the  
channel becomes free from bowlders. There is no doubt but that  
this seven-mile stretch of "devil’s jumps," as it is called, marks the  
_ trail of the fall’s retreat up the river. When we consider how slowly  
this retreat has been conducted (no appreciable change has been no- '  
ticed in the position of the falls since their discovery by white men gf 
about 1750) the vague length of time involved in this retreat must  
, appear very vast. Suppose we give to the falls a rate of retreat of  
one foot in one hundred years——it could hardly have been greater  
  . than this-and to the gorge a length of six miles—it is certainly  
; more--and we have over three million years as a minimum for the  
J age of the falls and gorge. With a less liberal allowance in rate and  
j a greater in distance, all within the limits of justification by facts,  
p and ten million years would not seem an extravagant estimate {OI' the  
l time in which this action has taken place.  
  Slow, however, as are these processes of river erosion, resulting in  
  their cutting down their beds and back their falls, they are rapid in  
_ comparison with the rate in another set of phenomena we will next  ‘-
consider.  
At the headwaters of Green river, on the borders of Pulaski and  f
l _ Casey Counties, there arises to the height of 1800 feet above sea  
; level-800 feet above the bed of the steam at its base-—a knob, known ·  
  as "G'reen River Knob." With its base in the Devonian black shalet  
.1 .