MUSEUM OF COMPARATIVE ZOOLOGY. 209 



There is yet another way of approachiug 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. Tliis may be set forth as follows. 

 The indentations on the surface which has been ex'oded 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 suflSciently hard 

 to make pei'ceptible 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 cleai"ly indicated as on the unweathered portions of Iron Hill. By 

 cai'efully 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 



