REVIEWS 57 l 



the value of these phases of action, and that in turn is dependent on 

 the duration and complexity of the glacial period. 



A strict discussion is constantly vexed by the question whether the 

 competency of the glaciers to erode is to be measured by the absolute 

 amount of work done by them during their existence, be it longer or 

 shorter, or by the ratio of the work they do to that which would be 

 done by the usual erosive agencies in the same area and for the same 

 time. Are we trying to determine the absolute or the relative ? The 

 latter is probably the truer basis of estimation in general, but both 

 have their special values. 



In the case in hand, such degradation of the tributaries at their 

 mouths as took place during the period of glaciation lowered the base 

 of reference by which the glacial erosion is measured, and a plus cor- 

 rection is required to give its absolute amount as already indicated. 

 The lowering of the gradients and the premature flattening of the 

 tributary basins, if uncorrected, leads to an erroneous projection of 

 lines across the valleys and requires a negative correction in absolute 

 measurement, and a more complicated and serious correction in relative 

 measurement. 



These suggestions do not cover the whole case, but they go as far 

 as is perhaps permissible in a review, indeed, not unlikely farther than 

 is warranted in the review of a paper based on fugitive studies that do 

 not claim an exhaustive character. The importance, or otherwise, of 

 the qualifications suggested depends much upon the duration and the 

 fluctuations of the glacial period and the ratio of the general glacial 

 action to the local valley phases. 



If there is any reference in the paper to similar discordances 

 between main valleys and tributaries not in any way connected with 

 glacial erosion, it escaped the eye of the reviewer. Such cases of 

 declared form exist and might naturally be expected to find at least 

 passing recognition in a paper founded on discrepancies of this kind, 

 the more so because in this neglected class also the main valleys are 

 discrepantly broad and deep and the tributaries cascade down into 

 them not unlike those described in the paper, though much less strik- 

 ingly. Probably the neglected case has little application to the Alpine 

 valleys discussed, and perhaps only an unimportant application to 

 those of Norway. The case referred to arises from changes of drain- 

 age, whereby large volumes of water are thrown into valleys that had 

 previously carried much less ; such cases, for example, as the Upper 



