212 E. C. ANDREWS. 



coherence of the textural units is gradually overcome and 

 differential motion, or flow, ensues. Even a crystalline 

 solid may be forced to flow under great gravitative stress. 

 As opportunity offers, such stream material will revert to 

 its solid condition, and a stream thus induced, and possessed 

 only of the atmospheric cover, may present the appearance 

 of a rigid solid at one place, a typical stream at another, 

 and a quasi-plastic compromise to gravity in the inter- 

 mediate region. Something akin to this state of affairs 

 apparently occurs in modern glaciers. In the glacial 

 "slacks " one gets the idea of a crystalline solid; below a 

 variable englacial surface there is apparently a plastic 

 response to gravity, while the main mass exposed to the air 

 is at least a compromise between these two types* Of course 

 this has reference only to glaciers of a definite size, or to 

 fairly large ice masses working along declivities or in 

 valley constrictions. Up to such stage an ice mass may 

 only present a feeble response to gravitative stress. (A 

 good example of the latter stage is the ordinary ice block 

 sold in towns. Very valuable information as to the ordinary 

 properties of ice — such as regelation — may be obtained from 

 a study of such ice cubes. Nevertheless, the inferences of 

 a student whose knowledge of ice was confined to an 

 acquaintance with such ice cubes would not be very valu- 

 able in any discussion as to the probable origin of fiord 

 basins by glacial action). 



Again, many potential streams may be conceived as 

 possessing various sets of textural units. Under one set 

 of conditions, flow may take place by differential movement 

 between one group of textural units, while under greater 

 gravitative stress, flow may be transferred to another set 

 of units. Thus a heap of bricks may form a gravity stream, 

 the individual bricks forming the textural units of such a 

 stream. Under increased pressure the bricks may be 



