184 WAVES OF SA^ND AND SNOW 



efforts to classify the materials whose drifting has 

 to be studied, classification by size being insuffi- 

 cient, owing to the fact that the greater mobility 

 of smaller particles may be compensated by higher 

 density or a more compact form. There is, how- 

 ever, a single constant which suffices for the classi- 

 fication of solid particles in the order of their 

 mobility of drift, i and this is their rate of subsi- 

 dence. All solid particles may thus be arranged in 

 three classes. There are first the least mobile 

 particles, whose rate of subsidence is so great that 

 they only travel on the surface and are never more 

 than momentarily suspended. In the desert this 

 order of material is represented by the gravel. 

 When dealing with the movement of detritus by 

 tides and waves it is represented by shingle. If 

 we care to study the way in which the winds of 

 autumn drift fallen foliage, we shall find that the 

 leaves of the plane-tree came in the same category. 

 Passing by for the moment the middle, or second, 

 class, we have as the third, or most mobile, par- 

 ticles those whose rate of subsidence is so slow 

 that they are held in suspension as long as there 

 is any upward movement of the encompassing fluid. 



^ See Geographical Journal^ April, 1908, pp. 421-2, remarks 

 by the author on Dr. J. C. Owens' " Experiments on the Trans- 

 porting Power of Sea-Currents." 



