Correspondence — " Reviewer." 191 



Here the upper surface of the ice slopes from C towards A, and the 

 flow is in the same direction. All the ice in the hollow is in 

 equilibrium and is urged in the direction of the arrow by that 

 portion which is above the lip A and extends towards C. Very little 

 force indeed is therefore required to raise the ice from B to A. The 

 ice will get thinner as it advances, and the slope of the upper surface 

 at each point will assume the angle required to deliver the proper 

 volume of ice at any cross section, aa, aa. 



If the glacier floor were immovable and the ice were frozen to it 

 the rate of motion at the bottom would be very small indeed if local 

 shear did not take place. We have proof, however, that the glacier 

 not only undergoes internal distortion, but that it frequently slips 

 over the floor as well. In Spitzbergen, where the ground is frozen 

 for hundreds of feet in depth, it is clear that ice flowing over such 

 a floor would freeze to it and drag it along. Under these circumstances 

 large masses of frozen gravel and sand would become incorporated in 

 the lower portion of the glacier together with boulders, and these 

 inclusions would be thawed out and redistributed when they came to 

 their journey's end. The 'pushing' effect of the ice front against 

 loose material as the glacier advances is no doubt considerable ; but it 

 is rather to the work effected by the debris-charged lower portion that 

 the transport of material is due. 



Mr. Crook suggests that the transport and lifting are more likely to 

 be the result of upward shear in the ice, say along the dotted line BD 

 in the figure, than along the floor from B to A. This seems very 

 unlikely. 



If the •Rhone Valley were filled with ice up to about the level of 

 the numerous hanging valleys the smaller glaciers from these valleys 

 would flow on to and over the surface of the main Rhone Glacier, 

 and subglacial moraine would thus find its way to the upper levels. 

 I do not think it at all likely, however, that bottom moraine would 

 find its way to the surface by upthrusts in the ice. 



R. M. Deeley. 

 Inglewood, Longcroft Avenue, 

 Harpenden, Herts. 



March 13, 1911. 



GEOLOGY OF PADSTOW AND CAMELFOED. 



Sin, — Iii the notice of the Geological Survey Memoir on the 

 Geology of Padstow and Camelford (p. 136) it was stated that the 

 two new maps which accompany that memoir " show, for the first 

 time, the divisions of Lower. Middle, and Upper Devonian ". So far 

 as the one-inch maps of the Geological Survey are concerned, this is 

 quite correct. Nevertheless, mention should have been made of the 

 fact that the main divisions of the Devonian rocks had been repre- 

 sented on a small map by Mr. W. A. E. Ussher (Trans. Roy. Cornwall 

 Geol. Soc, 1891, p. 273). That map was constructed from litho- 

 logical descriptions in De la Beche's Report, the data being interpreted 

 by Mr. Ussher from his intimate knowledge of the Devonian rocks in 

 South Devon. Although the relative position of the Meadfoot and 

 Looe Beds and of the Dartmouth Slates was not then understood, the 



