38 . CONTRIBUTIONS TO THE 



Such pot-holes exist in the anomalous position mentioned in 

 New Hampshire, where no present stream could by any possi- 

 bility be made to flow. One of them, measured many years ago 

 by Jackson, was eleven feet deep, four and a half feet in diame- 

 ter at the top, and two feet at the bottom, and when discovered 

 was filled with earth and rounded stones." 



The instance referred to above is in Grafton county, between 

 Grafton Centre and East Canaan. 



The whole account is no less wonderful than admirable, con- 

 forming wholly with what we have independently observed, with 

 the single exception of the reference to "shallow rapids." We 

 have become convinced that pot-holes are rarely if ever formed 

 except at the bottom of deep eddies and whirlpools, where there 

 is set up a continuous and nearly equable circular movement of 

 the water. Their formation in rapid and at once shallow cur- 

 rents could not occur, for the reason that the force of the stream 

 would continually wash down and away the stone tools which 

 might elsewise undertake the work. Besides, were Professor 

 Wright's assumption true, we should see the making of the char- 

 acteristic pot-hole going on under our very eyes. But this is 

 precisely what we do not see, and we are unable to assign such 

 examples as have come to our knowledge to any but a remote 

 era and to operations taking place at a very considerable if not 

 great depth of water. It is true that they may be still found in 

 shallow rapids, and even partially filled with pepples, but the 

 perhaps unintentional inference that they were now in process 

 of making does not appear to be warranted by observed facts. 



We venture to set down four important factors in the forma- 

 tion of the true pot-hole, to wit : i. Sufficient depth of water. 



2. A whirling and nearly equable movment of the current. 



3. Sufficient length of time. 4. Varying hardness of the rock 

 attacked, and hardness of the excavating tool. Under these 

 varying conditions the differing features of pot-holes, wherever 

 found and whether single or in groups, may be accounted for. 



