18G7.] LUBBOCK PARALLEL ROADS OF GLEN ROY. 91 



by the presence of a fluid which neither softens nor lubricates the 

 solid fragments. 



" The absolute pressure and friction are, of course, diminished in 

 water through buoyancy; and therefore the solid fragments are 

 more easily displaced by any disturbing force in water than they 

 are in air : but in the absence of disturbing force their positions of 

 equihbrium are the same in both fluids. 



" I may state that I have seen even sand stand under perfectly 

 still water at as steep a slope as in air. The grains were rough and 

 angular ; had they been rounded and smooth, the water would have 

 acted as a lubricant, and diminished the angle of repose. 



" I may remark that there is only one perfectly satisfactory way 

 of settling such questions as this ; and that is, by experiment. 

 " I am, my dear Sir John, 



" Yours very faithfully, 



'* W. J. Macqttorn Rankine." 



Now in the case of Glen Roy it is probable, from the occasional 

 appearance of the solid rock, that the sides were never very perpen- 

 dicular. However this may be, all observers are agreed that they 

 are now covered with a thick layer of angular detritus, and that the 

 solid rock rarely comes to the surface. 



It seems to me evident that the detritus stands at its angle of 

 repose, though this cannot be proved by calculation. The slope 

 of the sides of the valley appears, however, to be very nearly what 

 we should expect from the Table given above. Moreover it is, I 

 think, sufficiently obvious, from the circumstances of the case, that 

 the present slope is very nearly, if not exactly, that of repose. For 

 if not, it must be either at a greater or a less inclination. But loose 

 matter cannot stand at a greater angle than that of repose ; and 

 the sides of Glen Roy have evidently suffered little change since the 

 Glacial period. They cannot, therefore, be at a greater angle than 

 that of repose. Neither can they be at a less inclination ; for they 

 are formed of detritus which has come down from above by wea- 

 thering, while if the angle had been less, the detritus could not have 

 come down. 



Again, we know that when the lake stood at the level of 

 the two upper terraces, several streams ran into it; now, if the 

 inclination of the hill had been less than the angle of repose, the 

 matter brought down by these streams would have rested where they 

 entered the lake, and formed deltas. This, however, as Mr. Jamieson 

 remarks, and as his map shows, has not been the case. The angle 

 of repose of any given substance can, as I have already observed, 

 only be determined by experiment ; and it has been suggested to me 

 that I should endeavour in this way to determine whether the sides 

 of Glen Roy do stand at the angle of repose. It would, however, be 

 difficult, if not impossible, to reproduce the exact conditions ; nor, 

 indeed, is it necessary, since nature has herself tried the experiment 

 for us. It therefore seems to me evident that the sides of Glen Roy 

 do stand at the angle of repose ; and it is also clear that they must 



