MUSEUM OF COMPARATIVE ZOOLOGY. Zitat 
mass of which is at or near the freezing point, will, through the effect 
of pressure in promoting melting, gradually work down through the 
block without leaving any crevice behind it. It therefore seems not 
improbable that where the bit of rock which made the incision in the 
bed was small, the ice which held it moved more rapidly than the tool 
itself, and that the rate of movement of the cutting points was an un- 
known fraction of that at which the ice moved towards the margin of 
the glacial field. 
The time required for the passage of a length of the glacier equal to 
the distance from Iron Hill to Providence, is a matter of almost as 
much doubt as the amonnt of erosion which it accomplished. Our only 
possible source of information is found in the rate of movement of exist- 
ing ice streams. We are, it is true, tolerably well informed as to the 
speed attained at the extremities, and at various points on the surface, 
of valley glaciers of the Alpine type. Such observations as have been 
made on the larger ice streams in Greenland and Alaska show very 
clearly that the glaciers of these countries move far more swiftly than 
the better known streams of Switzerland and Norway. From the ob- 
servations which have been made on the arctic fields, it seems not un- 
reasonable to compute the motion of the New England ice at not less 
than twenty feet per diem, or say at the rate of about a mile a year, or 
from thirteen to fifteen years for the journey over the part of the train 
which we have endeavored to subject to computation. 
Although no kind of final value can be assigned to the results of the 
computations above given, it seems to me that they serve to indicate 
that the erosion accomplished by the ice while it lay upon the surface of 
this part of the continent was probably effected with great rapidity. 
The impression left upon the mind of the student who attentively con- 
siders and carefully reckons the more computable form of wearing which 
is brought about by scratching and polishing is to the effect that the 
surface must have worn downward at an annual rate which is certainly to 
be measured by inches, if not by feet. If after inspecting this evidence 
he will follow the course of the boulder train which we have been con- 
sidering, he will find that the quantity of the débris from the hill which 
it contains forces him to a similarly high reckoning as to the rate of 
the glacial wear. Even if from the data he obtains he should conclude 
that the estimate of the peridotite in the train which I have made is five 
or ten times too great, he will still be compelled to believe that the 
down-wearing took place in an exceedingly rapid manner. Minimizing 
the estimates in every possible way, in a manner which need not here 
