MUSEUM OF COMPARATIVE ZOOLOGY. 209 
There is yet another way of approaching this question of the rate 
of erosion brought about by the passage of a given amount of ice 
over the surface of the bed rock, —a method which is applicable in the 
study of many glaciated rock surfaces. This may be set forth as follows. 
The indentations on the surface which has been eroded by the glaciers 
are divisible into four classes: the pits which were left where disjointed 
masses of the rock were plucked out and borne away by the moving ice ; 
the grooves, or more or less distinct relatively broad channels, which 
have been carved in those parts of the rock made particularly accessible 
to erosion by the local softness of the material, or by the form of the 
surface, which led to local intensifications of the erosive work done dur- 
ing the passage of many successive cutting points composed of bits of 
hard rock held down upon the bed by the moving ice ; the scratches, 
which are distinguished from the grooves by the fact that they have 
been formed by the incisive action of a single point of hard material 
urged forward by the ice; and, lastly, the general polishing of the sur- 
face accomplished by the attrition of very small powdery fragments, 
which were not large enough to be fixed in the ice or sufficiently hard 
to make perceptible grooves, but which served to smooth the rock much 
as a polishing powder acts when rubbed upon a surface of metal by the 
human hand. 
For our present purpose we shall limit ourselves to that form of gla- 
cial wear which is effected through the action of the distinct scratches 
or indentations which are produced by the movement of a point of hard 
rock over the glaciated surface. On many rocks which are thus eroded 
it is possible to measure the length and breadth of these indentations, 
and to determine the relative amount of wear which is in this manner 
brought about. It is rarely the case that the evidence to this effect is 
so clearly indicated as on the unweathered portions of Iron Hill. By 
carefully examining the glaciated surface shown in Plate IV. we find that 
we may estimate the depth of these scorings at an average of one twen- 
tieth of an inch, and we may reckon the channels as covering one fifth 
of the surface, the intermediate spaces being occupied by parts of the 
rock which have been polished in the manner above described. The 
average length of these grooves appears to be about eight feet. It thus 
‘appears probable that while these rock fragments which made the incis- 
ions moved for the distance of fifty feet, they eroded somewhere about 
one twentieth of an inch from the surface of the rock which is the sub- 
ject of this computation. At this rate, while the cutting fragments were 
moving for the distance of a mile, the aggregate erosion accomplished 
