170 NEW YORK STATE MUSEUM 



One additional point should be made here. The condition of the 

 molecular film of water next any soluble substance must at least 

 be as complex as its molecular film in contact with the air. In the 

 film next the rock surface for instance, there are molecules of 

 water passing into the rock and molecules of rock passing into the 

 water. This film may have exceedingly tenaceous properties, and 

 the greater the difiiculty of removing it, of freeing the separate ions 

 or molecules in it through diffusion, the slower will be the process 

 of solution. When the solvent is thrown into rapid motion against 

 this film it not only helps to tear it away but some of its energy is 

 transformed into heat and electricity and before dissipation can 

 take place such transformation may very materially accelerate the 

 process of solution itself. In other words these forces may super- 

 saturate the film while they are in action and relief will be found 

 in forced diffusion aided by mass motion. We may therefore hold 

 for the present that dentpits are the result, in part, of differential 

 solution, due to differences in motion of the solvent; and in part 

 also to molecular abrasion and abrasion by molar particles of ex- 

 ceedingly small mass. 



Comparison of potholes, cupholes and dentpits. In pothole 

 formation we have large and comparatively constant vortexes which 

 are capable of moving large pebbles and of giving them a rotary 

 motion of considerable radius. These factors determine size. Grav- 

 ity plays a very important part in their formation for it determines 

 the direction of the cutting, holds the heavy tools against the 

 bottom with considerable force, and does not allow the vortex 

 motion to lift them and use them against the upper and older por- 

 tions of the excavation. The side cutting is continued by finer 

 material but the bottom cutting is so greatly in excess of it that 

 deep rather cylindrical holes with well rounded bottoms result. Sec- 

 tions along their axes will give somewhat U-shaped figures.^ 



In cuphole formation the vortexes are markedly intermittent in 

 action, if not reversed, are much smaller, more closely packed 

 together and incapable of handling heavy material. These factors 

 again determine size and also' number per unit of area. The part 

 played by gravity is not so important although it still loads the 

 vortex points, determines the direction of their cutting and keeps 

 their axes vertical. 



1 Giant kettles do not dififer much from potholes in the manner of their 

 formation though they do in the agent employed and in their size and 

 distribution. 



