SIXTH REPORT OF THE DIRECTOR I909 183 



fine and slightly raised lines which follow the rise and fall of the 

 dentpit surfaces. Such elevations on the general relief of this 

 surface are in no case greater than a fraction of a millimeter. 

 This type of solution is due to organic acids produced by resi- 

 dent organic forms, and is chemical in its nature. The cave 

 walls are protected from this agent, save in an indirect way, 

 for they are shielded from light. The lake waters themselves of 

 course hold carbon dioxid in solution and the supply is constant 

 and from various sources. It has no doubt markedly increased 

 the solvent power of Lake Champlain waters but its effects lie 

 within the limit of the 3 millimeters already set as the greatest 

 limit of quiet solution since the recession of the Hochelagan sea. 

 Effect of expansion of freezing water. We have already 

 called attention to this most powerful of all the disintegrative 

 agents now at work on Valcour island in our remarks covering 

 the features presented by plates 18 and 19. The vertical or 

 overhanging front of the cliff shown in plate 1 is very largely 

 ■due to this cause. In the Fourth Report of the Director of the 

 Science Division [N. Y. State Mus. Bui. 121] plates 5 and 6, will 

 be found two reproductions of this cliff from photographs which 

 I made in the winter of 1903. Both views were taken from the 

 ice surface shown in the foreground, and both contain a view of 

 some portion of the cliff shown in our present plate 1. The ice 

 curtains, which speak of frozen spray, have a very interesting 

 story to tell of cliff destruction. The effect of this agent is here 

 seen to be destroying caves instead of making them. The most 

 numerous and most marked changes in temperature occur on the 

 exposed faces of these walls. The waves break here and wet the 

 surfaces to great hights. The lake waters themselves are 

 rarely frozen over (save for a few days) until some time in Janu- 

 ary. The fall rains raise the level of the lake and many small 

 cave mouths are covered in consequence. The water in these 

 is never frozen. It is doubtful if the water in the innermost 

 recesses of some of the large caves is ever frozen. Once well 

 within one of the higher caves and we shall find wetting above 

 the water line to be but a fraction of the hight to which it is 

 wet outside. The dentpit etching on the inner wall faces speaks 

 of their comparative permanence, as do also the rounded edges 

 of the rock angles. On the outer face of these cliffs, where 

 blocks have been split by freezing water, no surfaces are etched 

 and all rock corners are sharp. At the south end of the island 



