270 Wisconsin Academy of Sciences^ Arts, and Letters. 



fashion, the fact that, not only may a glacial stream be parted and 

 an island be formed by a prominence projecting through the ice 

 and wedging it aside, or by valleys leading it around ; but also 

 that there may be such a combination of prominence and depres- 

 sion as — while not entirely parting the stream — to so thin the 

 ice passing over the prominence, that it shall be wasted and dis- 

 appear before it can join the main currents diverted on either 

 side ; so that there shall be a non-glaciated area, not on the sum- 

 mit of the prominence, but on its lower slope, and these Iconceive 

 to be the essential phenomena and elucidation of the Wisconsin 

 driftless area. 



