90 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 18, 



In order to realize what would take place under these circum- 

 stances we have but to remember what we have most of us seen in 

 many a Swiss valley. On each side rise steep cliffs, at the foot of 

 which is a larger or smaller "talus" of fragments detached from 

 above. The rock gradually weathers, and the fragments falling 

 down accumulate at the bottom, forming a heap, with its back 

 against the cliff and a slope towards the valley. The angle of this 

 slope varies according to the nature of the material, but under the 

 same circumstances is constant. 



This "talus" reaches to a greater or less height up the rock, and 

 it is evident that the time must eventually come when, from the 

 degradation of the cliff and the accumulation of the talus, the valley 

 will at length be bounded by equable slopes. 



The angle of these slopes will depend on the nature of the mate- 

 rials. 



Mr. Edwin Clark has kindly given me the following list, taken 

 from Kankine's ' Manual ' : — 



Dry sand, clay, and mixed earth from 37° to 21°. 



Wet clay „ 17° „ 14°. 



Damp clay „ 45°. 



Shingle and gravel „ 48° to 35°. 



Peat „ 45° „ 14°. 



Molesworth gives the following : — 



Gravel (average) 40°. 



Dry sand „ 38^ 



Sand „ 22°. 



Vegetable earth ... „ 28° 



Compact earth ... „ 50°. 



Shingle „ 39°. 



Bubble „ 45°. 



Clay, well drained „ 45°. 



Clay, wet „ 16°. 



There are, then, great differences between different substances, and 

 even between the same substance under different conditions. It 

 appears evident, however, that in still water the angle would be the 

 same as in air. 



As this is a point of much importance, and as I have found a very 

 general impression that the angle of repose in water would be dif- 

 ferent from that in air, I have put myself in communication with 

 Prof. W. J. Macquorn Rankine, and subjoin his reply: — 



"Angle of repose. 



" 59 St. Vincent Street, Glasgow. 



" My deab Sie Jokbt, — So far as my observation and experience 

 have gone, the angle of repose, or natural slope, of coarse detritus 

 consisting of fragments not capable of being softened or lubricated 

 by water is the same in stiU water as in air. 



" The obvious explanation of this fact is, that the ratio of friction 

 to pressure, upon which the angle of repose depends, is not altered 



