1 68 NEW YORK STATE MUSEUM 



provided that the water forced in to take the place of that whirled 

 away is itself unsaturated, and also provided that there is no limit 

 to the speed with which such molecules may become detached when 

 in actual contact with unsaturated films. 



In true differential^ solution we are confronted by a surface 

 whose relief speaks of different degrees of solubility due to differ- 

 ences in c'hemical or other character of the surface in question. 

 Our recent discussion however should make it clearly manifest 

 that we may have a relief that speaks clearly of differences in 

 motion (speed and direction) of the solvent and dentpits present 

 us with a relief perhaps in great measure so' formed. 



That a solid rock relief may express this difference in speed and 

 direction and motion of a solvent is also shown by numerous lime- 

 stone sheepbacks of this region, which exhibit deeply cut rill chan- 

 nels on their exposed surfaces. A typical boss of this character is 

 shown in plate 14. The channels may be seen to start from near 

 the top of a well rounded boss, where they have no appreciable 

 depth, take the natural direction of water flowing down the inclines 

 and sink to a depth of 15 centimeters below the surface before the 

 rill becomes 150 centimeters long. This is a rate of increase of 

 I in depth for every 10 linear. These channels are usually sub- 

 parallel and are often but 20 to 30 centimeters from each other. 

 The only abrading materials which are here to be obtained are 

 atmospheric dust and such particles as are separated from the rock 

 through solution. These rill channels are not wholly due to rainfall 

 but were in some measure due to melting glacial ice. Whatever 

 the source of the water there was a very evident increase in cutting 

 power due to change in velocity as the water passed down the in- 

 clines. Whatever allowance may be made for mechanical abrasion, 

 there will still be left a large part of the difference in depth which 

 is due to solution. We may hold this to be another example of 

 differential solution — due not to difference in character of the 

 rock mass but to difference in rate of motion of the solvent. 



Where the axes of dentpits are parallel to the bedding planes 

 their surfaces are often crossed by thin laminae containing a large 

 percentage of silica. If solution were the only process used in their 

 formation such laminae should form sharp projecting ridges, yet 

 differences in bedding are so little expressed through relief as to 

 be either not at all noticeable, or to be barely felt by the finger tips. 

 Mechanical abrasion thus seems to have been present at all times. 



The sharp edges of the intersections of confluent dentpits at once 



