SIXTH REPORT OF THE DIRECTOR I909 169 



demonstrate the fact that no abrasive particle used could have had 

 sufficient mass to enable it to escape from the vortex and cut across 

 this border. On the other hand it has already been pointed out 

 that the water during great storms is never noticeably discolored 

 where these dentpits occur, and that the lake bottom has been swept 

 clean of all fine material for considerable distances from the cliff. 

 The water is not bringing in silt to use in its attacks upon. the cliff, 

 but it is in fact removing, through the agency of lake currents, all 

 those particles which are fine enough to remain any considerable 

 time in suspension, and leaving them where the present lake does 

 not have the power to pick them up again. Thus we must conclude 

 that abrasive material of sufficient fineness to do work of this char- 

 acter is apparently conspicuous only by its absence. Some fine abra- 

 sive material must however always be present and to such inorganic 

 particles as may be brought, detached, or allowed temporarily to 

 remain, we must add such organic particles of the plankton as the 

 calcareous carapaces of microscopic Crustacea and the silicious 

 frustules of diatoms. 



There is a border land between solution and abrasion that is but 

 little understood. The points of contact between any two masses 

 may be considered as molecular rather than molar and differential 

 motion between the two masses might detach single molecules as 

 well as clusters of molecules. Increasing speed of molar motion 

 introduces new conditions and unexpected results. A tallow candle 

 is driven through an oak plank; water is made to cut rock in 

 hydraulic mining; onr great breech-loading guns, used in warfare, 

 are short lived because escaping gases driven with great velocity 

 cut away the metal around their breach mechanism. The energy oi 

 an ounce of hydrogen moving with the velocity of a thousand feet 

 a second is precisely the same as that of a steel ball of same weight 

 and velocity. A stream of molecules driven against a solid may 

 detach not only molecules but masses as well. We may get some 

 idea of this from tornado destruction. Note also the action of air 

 on swiftly moving meteoric bodies. We may then be justified in 

 concluding that there is such a thing as molecular abrasion, that it 

 is a fact to be dealt with in nature, and that it has played a part 

 in the formation of dentpits.^ 



1 For erosive power of escaping gases on guns see Ordinance and Gunnery, 

 by Col. O. M. Lissak, page 260. 



For statement that " The erosive effect of the gases appears to depend 

 more on their velocity than on the maximum pressure " see ibid, page 550. 



