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Ohio and the Upper Missouri. In these cases the great accessions of 

 water have broadened and deepened the main channels out of all con- 

 cordance with their tributaries. These at some distance back from the 

 trunk streams run in their old valleys slightly modified, but as they 

 approach the main valley, they rush down through new gorges, not, 

 indeed, as steep and picturesque as those of the Alps or of Norway, 

 but of like type. These have for some time been distinctly recognized, 

 and are in constant use as working criteria in discriminating earlier 

 and later systems of erosion, involving changed conditions. (See 

 " Further Studies of the Drainage Features of the Upper Ohio Basin," 

 Am. Jour., Sec. XLVII, April 1894, pp. 261-262.) 



It seems to have been demonstrated that in Norway the glacial 

 summit was some distance east of the present topographic divide and 

 that hence the Norwegian valleys were called upon to carry away an 

 amount of drainage greater than that which normally belonged to 

 them in preglacial times. This took the form of ice at certain 

 stages, and of water issuing from the edge of the ice field at other 

 stages. How far this may have contributed to the observed result it 

 is hard to guess without knowing more of the detailed history of the 

 glacial period in that region, but it illustrates the connection of this 

 mode of origin of discrepancies between trunk streams and tributaries 

 with similar discrepancies of a true glacial origin. It is probable that 

 even in the Alps, partly by topographic modification and partly by 

 superior condensation, glaciation has concentrated an exceptional 

 amount of drainage in the trunk valleys. 



It is not probable, however, that any or all of the modifications 

 herein suggested, or any others, seriously affect the representative truth- 

 fulness of the rough estimate of the superior erosive power of glaciers 

 founded on discordance between the tributary hanging valleys and the 

 glaciated trunk valleys. The paper is a valuable contribution to the 

 doctrine of glacial erosion, and is likely to be the more influential with 

 those holding opposite views because it comes from one who has here- 

 tofore held a conservative position on the subject. 



The reviewer does not, on first reading, sympathize fully with the 

 effort of Professor Davis to extend the analogy of river erosion so 

 unreservedly to glacial work as done in the paper. The analogy is 

 truer in gross externals than in refined analysis. A river erodes by 

 virtue of its pressure and momentum, scarcely at all by rigidity. Very 

 largely its work is done by the striking force of particles driven rapidly 



