160 Scientific Intelligence. 



hardly be used, as they lately have been by Russell (Science, 

 Majr 20, 1904, 785), to throw doubt on the glacial origin of 

 normal cirques and hanging valleys. It may be difficult to 

 explain the imitative forms, but as the matter now stands, it is 

 their explanation that is deficient, and not the explanation of the 

 normal forms by glacial erosion as stated by Gannett and Gilbert. 

 True, the trunk valleys are often so much deeper than the hang- 

 ing lateral valleys as to make it very difficult indeed to ascribe 

 their excess of depth to glacial erosion ; yet the hesitation that 

 one may feel here is more likely to be based on a conservative 

 habit of thought than on direct argument ; for the valleys are 

 certainly valleys of erosion, and when it is once shown that 

 glaciers are effective erosive agents, it does not appear more 

 unreasonable to ascribe the great trough valleys to the work of 

 frozen water than to that of molten water. It is pointed out by 

 Gilbert that the main valley sides are relatively smooth and 

 trough-like, sometimes exhibiting details of form that suggest 

 the action of an eroding agent which moved in nearly horizontal 

 lines, and thus contrasting strongly with the ravined valley sides 

 of non-glaciated mountains, where the agent of erosion has mani- 

 festly worked on down-slope lines. The extension of these main 

 valleys across the coast plain leads to the conclusion that their 

 erosion was accomplished by the larger glacial streams in conse- 

 quence of their relatively great velocity ; thus agreeing with the 

 opinion reached by Richter for the over-deepened valleys of the 

 Alps. 



The existing glaciers are described and illustrated in much detail, 

 with especial reference to their recent variations. It is shown 

 by a careful comparison of all available descriptions, maps and 

 photographs, that the changes of the glaciers have been singularly 

 discordant during the past century, and the discordance is held 

 to be too great to be explained by lagging. A most ingenious 

 suggestion is then made that under certain changes of mean tem- 

 perature, one set of glaciers might be caused to advance while 

 others near by would be compelled to retreat. Glacial students 

 who visit Alaska during the present century thus have a pleasing 

 problem set before them for solution. 



The volume closes with some general conclusions as to glaciers, 

 in which we find an altogether new view as to the possibilities of 

 glacial erosion beneath the sea. It has been recognized in recent 

 years that the invasion of once-glaciated valleys by the sea was 

 not, as it has usually been regarded, an evidence of submergence, 

 for heavy glaciers can certainly erode beneath sea level. Gilbert 

 now gives reasons for thinking that heavy tidal glaciers are not 

 effectively buoyed up by the water that they enter, and that they 

 erode beneath sea level about as effectively as on a land surface. 

 If this should be fully demonstrated, it would lead to a radical 

 change of opinion regarding the changes of level indicated by 

 fiord coasts. " Plucking " is looked upon as of great importance 

 in glacial erosion. w. m. d. 



