254 THE HISTORY OF THE NIAGARA RIVER. 



Ijoles, even in rocks that are hard. These holes are called technically 

 "pot-holes," and there is much to commend the sugorestion that the 

 excavation witliin tiie pool is essentially pot-liole work.* 



The process which I have described is tiiat which takes place in the 

 central part of the Horseshoe Fall, where the ;^reatest body of water 

 is precipitated. At the margin of the Horseshoe, and also at the 

 American Fall, in which places the body of falling water is much less, 

 the process is ditterent. There is there no i)Ot-hole action and no pool. 

 The fallen blocks of limestone form a low talus at the foot of the cliff, 

 and upon them the force of the descending water is broken and spent. 

 Such of you as have made the excursion through the Cave of the Winds 

 will recall that though for a few steps you traveled upon an undisturbed 

 rock stratum, one of the layers of the Clinton group, the greater part 

 of the journey lay across large fallen blocks of limestone, irregularly 

 heaped. Where, then, the volume of falling water is relatively small, 

 the great bed of shale below the Clinton ledges plays no part, and the 

 rate at which the limestone breaks away is determined purely by the 

 rate of erosion of the shale bed lying just beneath it. 



The ditference between the two processes is of great importance in 

 the present connection, because the two rates of erosion are very 

 different. 



I am fully aware that this sketch of the cataract's work is not a satis- 

 factory explanation of the mode of recession, but it yet serves a present 

 purpose, for it renders it possible to point out that the rate of recession 

 is attected by certain factors which may have varied during the early 

 history of the river. We see that the process of recession is concerned 

 with a heavy bed of hard rock above, with beds of softer rock beneath, 

 with the force of falling water, and possibly, also, with the solvent 

 power of the water. 



Concerning each of these factors a number of pertinent cpiestions may 

 be asked, questions that should certainly be considered, whether they 

 are answered or not, before any solution of the time problem is regarded 

 as satisfactory. To illustrate their pertinence, a few will be propounded. 



Question 1. Does the limestone vary in constitution in different parts 

 of the gorge? If its texture or its system of cracks and joints varies, 

 the process of recession may vary in consequence. 



Question 2. How does the limestone bed vary in thickness in ditter- 

 ent parts of the gorge? This question is easily answered, for at all 

 points it is well exposed for measurement. 



Question 3. How is the thickness of the limestone related to the rate 

 of recession ? This is more difticult. The debris from a very thick bed 

 of limestone would oi)pose great resistance to the cataract and check 

 its work. The debris from a very thin bed would attbrd small and in- 

 efficient pestles for pot-hole action, and might lead to a slow rate of 



* I am iutlebted for this suggestion to Mr. W J McGee. 



