MUSEUM OF COMPARATIVE ZOOLOGY. 211 



mass of wliich is at or near the freezing point, will, throngli 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 ifhe 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 amount of erosion which it accomplished. Our only 

 possible source of information is found in the rate of movement of exist- 

 ing ice streams. ^Ve 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 Xew 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 debris 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. jNIinimizing 

 the estimates in every possible way, in a manner which need not here 



