276 E. C. ANDREWS. 



And this we think is the point which, properly apprehended, 

 would explain most of the difficulties experienced in the 

 study of glacial problems. But of this more anon. 



Summary of flood and drought stream action. — The 

 vertical strength of stream corrasion is not related in a 

 simple manner to its increase in velocity, hence a channel 

 bed approximately uninterrupted at one period, would if 

 visited by long heavy flood action, become diversified by 

 huge interruptions, such features being accentuated in 

 regions of youthful topography. Conversely, immediately 

 after a great reduction in stream velocity the locations at 

 which the drought-stricken stream reveals its maximum 

 incompetence would be those of maximum interruption of 

 channel bases. 



Special application of stream principles to glaciers.— 

 In the present chapter an endeavour has been made to 

 explain the significance of the forms found in regions of 

 former intense glaciation, simply from a knowledge of the 

 mechanics of streams. The deductions are based on the 

 belief that in the regions about to be considered, several 

 strong glacial floods acted along canons and other valley 

 types originating in ordinary stream action. This assump- 

 tion of such pre-glacial valley formation is necessitated by 

 the knowledge that glaciated and non-glaciated topo- 

 graphies exist in close association along the valleys of 

 California (for example, that of the Merced below Yose- 

 mite), of Western New Zealand, of Australia, and other 

 localities where glaciated profiles upstream gradually pass 

 into typically non-glaciated profiles lower downstream, and 

 where the non-glaciated country situated immediately 

 downstream of the traces of glaciation is in such a stage 

 of development as to strongly suggest that the work of 

 glaciation appears to have been confined to profound local 

 modification of the valley contours, without alteration of 

 the main pre-glacial valley directions. 



