364 THE JOURNAL OF GEOLOGY. 



reduced by increase of width and depth ; though if (as is prob- 

 able) erosion varies more nearly with the weight than the veloc- 

 ity, its amount will increase absolutely, and the expanded valley 

 will tend in a sti-onger degree than that measured by the ratio 

 of the inverse volumes to assume the general form characteristic 

 of contracted glacial gorges. As in the contracted gorge, too, 

 lateral effectiveness will remain predominant ; but the effective 

 energy of the glacier will be mainly concentrated upon the ob- 

 structive angles, spurs, and cusps of the irregular water-carved 

 walls, and the removal of these and the rounding out of the am- 

 phitheatre will be in the first work of the glacier. Again, the 

 partial rigidity of the ice-mass will lead to culmination of pres- 

 sure about the distal extremity of the amphitheatre, and to 

 consequent extension of its boundaries beyond the confluence of 

 the tributary by which its water-fashioned prototype was origin- 

 ated. 



It follows that glaciated amphitheatres may be merely water- 

 carved valley expansions modified by temporary ice-action into 

 regularity of contour (as are, for instance, those of the Faeroe 

 Islands ^ ), and that they do not necessarily argue profound glacial 

 erosion. 



VI. 



Summarizing the chief effects of the several agencies involved 

 in the development or the characteristic features of glacial 

 canons, it appears that temporary occupancy of a typical water- 

 cut caiion by glacier ice will (i) increase the width, (2) change 

 the V to a U cross-profile, (3) cut off the terminal portions 

 of tributary canons, and thus relatively elevate their embouch- 

 ures, (4) intensify certain irregularities of gradient in the canon- 

 bottom, (5) excavate rock-basins, (6) develop amphitheatres, 

 and, in general, transform such canon into an equally typical 

 glacial canon. It follows that these features do not necessarily 

 imply extensive glacial excavation or indicate that glaciers are 

 superlatively energetic engines of erosion. 



W J McGee. 



'J. Geikie, " Geology of the Faeroe Islands," Trans. Roy. Soc, Edin., 1882. 



Ij 



