294 A NATURALIST IN HIMALAYA 



the bed of the stream itself was rolling. As the sand- 

 dunes of the desert roll in waves before the wind, so 

 was the sandy bed of the stream rolling in waves 

 before the waters. Parallel folds of sand, regular 

 dunes in miniature, crossed the bed of the stream. 

 Facing up the stream each fold presented a gradual 

 and shelving slope, while facing down the stream the 

 slope was steep and perpendicular. It is just the 

 same as in the desert dune. When the stream flowed 

 slowly on, layer after layer of sand was rolled up 

 the gentle slope until it reached the summit of the 

 fold, when it poured down over the perpendicular fall. 

 Here, sheltered from the current by the rising summit 

 of the fold the sand-grains come to rest, but layer 

 after layer pours down upon them, each layer covering 

 the preceding one. The trough in front of the fold 

 thus rises higher and higher as the sand pours down 

 from above, until at length what was once a calm 

 recess has risen to be the summit of a new fold. And 

 thus the folds move onwards by the downpouring sand 

 always building up a new summit in the recess in front 

 of the existing summit. They move by the same 

 mechanism as that which sweeps the dunes across the 

 desert. Each sand-grain is first a part of the gentle 

 slope, then of the summit of the fold, then of the 

 perpendicular fall, then lying motionless for a time 

 in the calm recess, it is engulfed beneath the down- 

 pouring sand until, as the wave moves onward, it is 

 again exposed, again rolled on by the untiring stream, 

 to again build up fold upon fold, and to repeat 

 hundreds of times a day this same eternal roll till 

 at length it will rest in peace beneath the ocean. 



It was instructive to watch the motion of these 



