1883.] 



On the Formation of Ripple-mark. 



25 



crest, and the point of growth oscillates on each side of the crest ; 

 the end of the tail flips backwards and forwards. Next the end of 

 the tail spreads out laterally on each side, so that a sort of mushroom 

 of ink is formed, with the stalk dancing to and fro. The height of 

 the mushroom is generally less than a millimetre. 



Fig. 3. 



Fig. 3 is the best representation I can make of this appearance, 

 which I shall call an ink mushroom. The first of these figures gives 

 the extreme of excursion on one side, the second the mean position, 

 and the third the extreme on the other side. The figures show the 

 state of affairs when the oscillation is very gentle, so that the 

 amplitude of oscillation of the main body of water is small compared 

 with the wave-length of the ripple-mark. The elongated hollows 

 under the mushroom are the vortices, and the stem is the upward 

 current. If the ink be thick these spaces are clouded, and the 

 appearance is simply that of an alternate thickening and thinning of 

 the ink on the crest. When one is familiar with this motion, after 

 examining it carefully with gentle oscillation over ripple-mark of some 

 size, the same kind of dance may, I think, be detected in the stage of 

 ripple manufacture after the sand has curdled into elongated flocculent 

 masses. 



The oscillations being still gentle, but not so gentle as at first, 

 streams of ink from the two mushrooms on adjacent crests creep 

 down the two slopes into the furrow between the adjacent ridges, 

 and where they meet a column of ink begins to rise from the part of 

 the water whose mean position is in the centre of the furrow. The 

 column is wavy, and the appearance is strikingly like that of smoke 

 rising from a fire in still air. 



The column ascends to a height of some 5, 10, or perhaps 20 

 times the height of the ripple-marks, according to the violence of 

 the agitation. It broadens out at the top on each side and spreads 

 out into a cloud, until the appearance is exactly like pictures of a 

 volcano in violent eruption ; but the broad flat cloud dances to and 



