260 E. C. ANDREWS. 



shall be unable to drag its load over the live rock structures 

 forming these heads. Thus a whole channel constriction 

 may be occupied by a deep basin, or several constrictions 

 and several points of stream confluence may be joined up 

 by one large basin as an end result of such head ward 

 recession of basins and allied forms. 



This method of corrasion must hold also for material 

 which is solid under ordinary conditions and pressures, but 

 which is forced to flow under great pressure. With the 

 increase of pressure comes increased mobility; this 

 increased mobility is not related simply to corrasion, hence 

 exceptionally large volumesof such material when flowing on 

 steep channel slopes and through decided constrictions, will 

 accomplish relatively great vertical and lateral corrasion. 



Decrease of stream volume implies the shrinkage of the 

 lateral, vertical and longitudinal measures of stream 

 strength. This in turn implies readjustment of channel 

 grades and stream incompetency at just those locations 

 where the great volume had revealed its greatest energy. 

 The quantitative value of such vertical and lateral cor- 

 rasion could be estimated. 



Part II. 



Glaciers are Gravity Streams.— Is a glacier a gravity 

 stream ? Is it influenced by increase of volume, by declivi- 

 ties, and by channel constrictions as ordinary streams are ? 

 Is it susceptible to flow under these conditions, the mobility 

 so gained increasing in same ratio, simple or complex, with 

 the pressure ? Does it follow the lines of least resistance 

 or lines of quickest descent as ordinary streams do ; does 

 the " law of the cross-sections' ' hold in its case, or does it 

 depend for its motion on physical conditions which differ 

 qualitatively from those governing ordinary stream motion? 





