Waves of sand and snow and the eddies which make them . s in the order of theirmobility of drift,i and this is their rate of subsi-dence. All solid particles may thus be arranged inthree classes. There are first the least mobileparticles, whose rate of subsidence is so great thatthey only travel on the surface and are never morethan momentarily suspended. In the desert thisorder of material is represented by the dealing with the movement of detritus bytides and waves it is represented by shingle. Ifwe care to study the way in which the winds ofautumn drift fallen foliage, we shall
Waves of sand and snow and the eddies which make them . s in the order of theirmobility of drift,i and this is their rate of subsi-dence. All solid particles may thus be arranged inthree classes. There are first the least mobileparticles, whose rate of subsidence is so great thatthey only travel on the surface and are never morethan momentarily suspended. In the desert thisorder of material is represented by the dealing with the movement of detritus bytides and waves it is represented by shingle. Ifwe care to study the way in which the winds ofautumn drift fallen foliage, we shall find that theleaves of the plane-tree came in the same 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 slowthat they are held in suspension as long as thereis any upward movement of the encompassing fluid. ^ See Geographical Journal^ April, 1908, pp. 421-2, remarksby the author on Dr. J. C. Owens Experiments on the Trans-porting Power of 185 SNOW-WAVES AND SNOW-RIPPLES 187 The surface layers of a deposit of such material alsodrift superficially before even a very slow the case of aeolian transportation this third otmost mobile class of particles is dust. In the caseof the movement of detritus by water it is mud. The middle, or second, term of the series com-prises particles which subside at a moderate rate,and material of this order is called sand, whetherit be found on land or under water. It has indififerent places a different mineral composition, buton the whole its chief component is quartz, whosehardness and texture are such that when thediameter of the particles is from about 5V toabout 200^ of an inch their mutual attrition bycollisions when drifting is very slow. •When dry sand is drifting before the wind on abeach where stones lie scattered on the surface,a single tongue of loose sand deposits behind eachprojecting stone, with a sharp crest
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