SIXTH REPORT OF THE DIRECTOR I909 I79 



solution, but we are on very uncertain ground when the abrasive 

 material used is as fine as it must be in this case ; and also when its 

 particles are so much freed from the influence of gravity. The 

 practical absence of evidence for differential solution on dentpit 

 surfaces, however, is a very good indication that molar abrasion is 

 here still the most important factor. We may allow it at least 

 two thirds of the 2^^ millimeters noted. The remaining 9 milli- 

 meters will then become an index of an utmost limit for the amount 

 of erosion accomphshed by molecular abrasion, solution in presence 

 of a rapidly moving solvent, and solution in a quiet solvent or 

 where molecules and ions can escape from the contact films only 

 by diffusion or movement induced by gravity. 



As the rapidly deepening rill channels on roche moutonnee slopes 

 are by themselves strong evidence of the powerful influence of a 

 rapidly moving solvent, and as our previous discussion shows that 

 we should expect it to be such in both this type and in dentpit 

 formation, we may assume that vortex motion through molecular 

 abrasion, through bringing the greatest volume of water in contact 

 with a given surface in a given time, through forceful detachment 

 of portions of saturated contact films and through various trans- 

 formations of energy, is responsible for at least two thirds of this 

 9 milHmeters, and that quiet normal solution, relying on simple 

 gravity or diffusion to further liberate the molecules which have 

 passed into the contact film of the solvent, is responsible for not 

 more than 3 millimeters of a surface on which it has been acting 

 for 50 centuries or more, which is the time we may temporarily 

 assign for the age of Lake Champlain. 



While, therefore, solution may have played a very important part 

 in the formation of these caves, it was very certainly not solution 

 by the waters of our present Lake Champlain. 



Additional evidence as to the effects of solution is to be found 

 in Sloop bay. At the foot of a cliff which terminates the horizontal 

 depth of the bay there is exposed a wide glaciated shelf which 

 dips gradually southerly and enters the present waters of the lake. 

 The higher portions of this shelf are still covered by till and show 

 the usual scratched and polished surface of the rock, when un- 

 covered. That portion which has longest been uncovered, save at 

 highest water of spring floods, has been acted upon by expansion 

 of water freezing in its joints and bedding planes, and large blocks 

 have been loosened to be pushed about by shore ice or dragged sea- 

 ward by ice blocks during the breaking up of the lake in the spring. 



