Davis.] 20 [May 3, 



belief that without glacial action the Alps would have no valleys. 

 Further opportunity for the choice of a ready-made opinion is 

 given in the quotations and abstracts below from the writings of a 

 number of geologists of different countries ; it certainly does not 

 lead to conviction, and the student must remain unsettled in his 

 mind until he can go behind the authorities and come to observa- 

 tion and argument for himself. 



How does it happen that there are so many contradictory 

 answers to the question ? We cannot lay all the disagreement to 

 incorrect observation or imperfect conclusions, but must con- 

 sider it to arise partly from the essential difference of glacial 

 action in different regions. I believe that the following summary 

 — an eclectic statement of the 'subject — may reconcile some of the 

 divergent views and explain some apparently conflicting opinions. 

 The observations and arguments on which this summary is based 

 will be referred to below. 



Glacial erosion was greatest near the centres of glacial disper- 

 sion, where the ice acted for the longest time, and where its 

 thickness and velocity were greatest; here it succeeded in 

 scraping away all of the rubbish of preglacial disintegration and 

 rubbing down the solid rock below in some places for a moderate 

 number of feet; here glaciers lowered the hills and deepened 

 the valleys on which they moved. In their middle course, the 

 extended ice-sheets were sufficiently occupied in carrying forward 

 for a short distance the loose material that they found ready 

 made, without attempting to wear away much of the rock below, 

 except from projecting knobs; here they generally lessened the 

 roughness of the country by rubbing down the ledges and filling 

 the valleys. Near the broad margins, where melting equalled or 

 overcame supply, where the ice was thin and slow-moving, and 

 its under part was clogged with detritus, the ice-sheet acted more 

 in the way of deposition than destruction, and as a rule failed 

 to rub away the loose soils and gravels over which it advanced ; 

 here the principal effects are found in a concealment of previous 

 lines of drainage by irregular accumulation of drift. 



It should be noted also that glacial erosion was strongest during 

 the early phases of its action, when it had loose surface detritus 

 in large quantity to deal with ; and that later, when the solid 

 rock was reached, further change must have progressed with 

 increasing slowness. 



