268 E. C. ANDREWS. 



Another significant fact noticed in connection with these 

 Sierran observations is the occurence of almost stagnant 

 glacierets at the heads of these deglaciated valleys. One 

 of these small ice masses showed beautiful bands in the 

 small ice cliff at its foot. Such a glacier must have been 

 practically inert. Nevertheless all over the valley sides 

 lower down were grooves and markings of such size and 

 shapes as to suggest intense and variable motion for the 

 former and larger glaciers. Thus increased volume of ice 

 all other things being equal, apparently implies an increased 

 plasticity for glaciers, and hence a great increase in their 

 corrasive power. 



Further notes on the action of a crystalline solid (ice) 

 forced to flow under gravity.— Firstly, the topography the 

 ice stream may start upon is a chance one. The stream 

 may utilise and make a selective action among local struc- 

 tures, but so far as its origin was concerned it simply 

 grew, as its material (snow flakes) may have fallen from 

 the sky; by the action of gravity it became a stream, 

 and up to this stage it had no choice of topographic or 

 structural environment. In brief, the excessively mobile 

 stream could not elect to act on incoherent structures only, 

 such, for example, as sand, any more than the excessively 

 large stream of sluggish habit could elect to flow over the 

 strongest structures only. An important point is thus 

 disclosed. Since the structures are practically the same 

 over which the mobile and sluggish streams work, there 

 may come a time when the limit between stream and 

 structural strengths may be exceeded, and then the impact 

 of the enormous stream mass (glacier) will overcome the 

 strength even of important channel structures, the while the 

 very mobile stream will corrade mainly by abrasion. 



Now, in the case of a solid forced thus to flow, both its 

 falling action over ledges and its rolling or sliding down of 



