RECENT AND FOSSIL RIPPLE-MARK. 
35 
marks show crests, which, in examples produced under different 
conditions, range from sharply angular to nearly flat or rounded, 
so interference ripples exhibit corresponding variations of type. 
Successive observations of one set of interference ripples, con- 
tinued for three days, have also shown that aging of the impres- 
sions, under the influence of a shifting direction of the wind, 
tends to alter angular partitions to rounded ones, thus producing 
patterns resembling the “tadpole nests” of Hitchcock. The 
comparative rarity of the so-called “tadpole nests” affords strong 
evidence that they are the product of the wind rather than of 
tadpoles. I have observed many scores of ponds with tad- 
poles, but have only once seen the dimpled surface called “tad- 
pole nests” by Hitchcock associated with tadpoles. Interference 
ripples are much less common than the wave-like type of ripple- 
mark so frequently seen in shallow water. They are developed 
under shallow water in which the ordinary wave generated 
by the action of the wind on the surface is split up into two or 
more sets of oscillations moving in different directions. The 
gradual movement of fine sediments by such conflicting oscil- 
lations results in the coarse cell-like structure shown in Plate 
XXII. The interruption or breaking up of regular wave oscil- 
lation, which occurs about the ends of bars, piers, or stranded 
logs, affords favourable conditions for the development of inter- 
ference ripples. When the bottom is favourable interference 
ripples are generally found in the shallow water of such localities. 
The cast of the interference ripples shown in Plate XXIII C was 
made in water about 6 inches deep and protected on all sides by 
stranded logs from any but the lightest wave action. The 
photograph shown in Plate XXII was taken from a plaster 
mould of interference ripples made under a few inches of water 
at Britannia bay, Ontario, where the regular wave oscillations 
are interrupted by the pier and stranded logs. This picture 
indicates the close relationship of the interference ripple-mark 
to ordinary wave ripple-mark. The lower half of the picture 
shows a type belonging more to the ordinary oscillation ripple 
than to the interference form. The upper half shows the 
troughs of the oscillation ripples broken up by a series of more or 
less discontinuous partitions giving the effect of a series of four- 
