176 NEW YORK STATE MUSEUM 



widths of about 10 millimeters and depths of from 1 to 2 milli- 

 meters. The presence of dentpits shows us at once that these 

 softer layers have not been cut away by simple solution but 

 that erosion at the hands of vortex motion has played an im- 

 portant part in the process. Remembering also that dentpits 

 show absence of cutting" vortex points loaded with sediment 

 by gravity, we find them in this case indicative of surfaces 

 which can not well have been upper surfaces. The two sur- 

 faces on which these dentpits appear are both on the under- 

 side of the specimen as it is shown in plate 15, or on the broad- 

 est face of the present three-sided interior prism and are only 

 separated from each other by the thin silicious sheet of the 

 middle bedding planes. As in this plate the specimen is rest- 

 ing on the remnants of its older prismatic face of greatest area, 

 which was the position in which its center of mass would be 

 lowest, it would appear that these surfaces were not only under- 

 surfaces when found but that they had been such during the 

 whole age of Lake Champlain. These two dentpitted surfaces of 

 the softer beds of the stone have been cut by abrasion and solu- 

 tion together until they are now 27 millimeters distant from the 

 outer edge of the harder layers or from the probable surface of 

 the stone as the last ice sheet left it. This aspect of the speci- 

 men is shown in plate 16, figure 1. 



The surfaces of the soft beds which are opposite the sur- 

 faces just described are indented with numerous, more conical 

 pits, measuring from 2 to 3 millimeters in diameter and having 

 about half that depth. The cutting here is of the type effected 

 by vortexes whose points are down, and kept filled with sediment 

 by gravity. Though the dimensions of these cuts are so small, 

 they have lowered this surface of the specimen some 55 milli- 

 meters. As cutting of this type can take place only in a down- 

 ward direction the surfaces of the specimen on which they ap- 

 pear must have been upper surfaces. These are the surfaces 

 uppermost in plate 15 and the greater rapidity of the cutting on 

 this portion of the specimen only served to lower its center 

 of mass the more and make its equilibrium still more stable. 

 These surfaces have very probably been upper surfaces ever 

 since the cessation of motion of the last ice sheet. This as- 

 pect of the specimen is shown in plate 16, figure 2. 



The character of the two soft layers does not change on passi- 

 ing from one side of this stone to the other and the difference 



