210 BULLETIN OF THE 



through the formation of scratches would amount to about one hundred 

 twentieths of an inch, or to near five inches, and during a journey of 

 these catting fragments to the distance of fifteen miles, to about 

 seventy-five inches. Allowing, as before estimated, that the plucked 

 out material carried away in the form of boulders amounts to as much 

 as one fifth of that removed in the other forms of erosion, the aggre- 

 gate wear may be estimated during the time when thirteen miles of ice 

 was passing the point as somewhere about seven or eight feet. 



There is a manifest source of error in the computation last given, 

 which arises from the fact that no account is taken of the form of 

 erosion which occurs between the scratches, which results in the produc- 

 tion of a smooth surface, or in scorings which are so delicate as not to 

 make an impression on the eye. As at least three fifths of the surface 

 is of this charactei", it appears to me that we must reckon the abrasion 

 due to the rubbing of the rock by very fine particles of deti'itus to be 

 about twice as effective in removing material as that which produces tlie 

 scratches. Were tliis not the case, the discernible indentations would 

 occupy a lai'ger part of the field. AVe therefore see that to the com- 

 bined scratching and polishing actions we may perhaps reckon a total 

 lowering of the surface during the passage of the ice over the distance 

 between Iron Hill and the town of Providence at about twenty feet. 



The large proportional share taken in erosion eflected by the so called 

 polishing woi'k done by the glacier lias been generally overlooked. It 

 appeai-s to me that on most hard rocks it has been the efficient means 

 "by which they were worn away during the passage of the glacier over 

 their surfaces. A careful examination of any completely smoothed ma- 

 terials, such as our harder granites or the denser clay slates, will make it 

 evident to the eye that the most of the waste which was removed from 

 the surface, at least during the last stages of the glacial erosion, was 

 taken away in the form of a very fine powder, the so called glacial flour, 

 which we know by many observations is likely to be carried to an indef- 

 inite distance by the sti'eams which flow from the glacial area. In tins 

 way, we can account for the fact that tliis fine detritus has generally 

 disappeared from the districts about the ice front. 



There is yet another evident doubt which serves to diminish the value 

 of the above given rude approximations to the rate of glacial wearing. 

 This is due to the fact that we do not cleaHy know that the hard points 

 which effected the incision of the scratches were firmly held in the ice 

 as they were urged over the surface of the bed rock. It is well known 

 tliat a wire suspending a weight and resting upon a block of ice, the 



