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 cutting 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 character, it appears to me that we must reckon the abrasion 
due to the rubbing of the rock by very fine particles of detritus to be 
about twice as effective in removing material as that which produces the 
scratches. Were this not the case, the discernible indentations would 
occupy a larger part of the field. We 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 effected by the so called 
polishing work done by the glacier has been generally overlooked. It 
appears 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 renioved 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 streams which flow from the glacial area. In this 
way, we can account for the fact that this 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 clearly 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 
that a wire suspending a weight and resting upon a block of ice, the 
