18 
MUSEUM BULLETIN NO. 25. 
ridges and their destruction by the automatic variation of the 
current velocity above and below the critical velocity for ripple- 
mark formation. In the first stage of the cycle of ripple-mark 
formation observed, a strong current flows over a smooth un- 
rippled sheet of sand; then ripple ridges form extending across 
this stream 4 to 6 or 7 feet apart. The position of these was very 
apparent through the breaking of the thin sheet of water over 
them. Immediately after the formation of these ripples of large 
amplitude the slackened current produced intermediate ripples 
at much closer intervals. These quickly resulted in a further 
slowing down of the current by the damming effect of the ripple- 
ridges. The reduction of velocity was accompanied by an in- 
crease of the volume held on the slope until the current some- 
where, usually in upper part of the grade, breached a ripple. 
This, of course, increased the pressure on the next ripple below 
which gave way and thus the constantly augmenting volume of 
water swept away all of the closely placed ripple-marks leaving 
a smooth slope of sand ; then the process was repeated beginning 
with the formation of the widely spaced ripples. At this peculi- 
arly favourable locality it was thus possible to directly observe 
under natural conditions the alternate formation and sweeping 
away of ripple-ijiark through a slight reduction followed by an 
increase in current velocity. 
Plate III B affords an interesting example of the record, 
which is sometimes left on the banks of streams, of the work of a 
current with decreasing velocity. On the left hand side of the 
picture is seen well-developed ripple-mark which was formed 
when the stream was at flood stage and the current correspond- 
ingly swift. When the water had fallen to the level of the top 
of the terrace which truncates the ripple-mark pattern, the 
velocity became too low to form ripple-mark and it began to 
erode and destroy the pattern previously formed over the whole ' 
of the slope shown in the picture. The remaining stages of the 
retreat were marked by six periods of comparatively rapid 
withdrawal of the water which are indicated by the small ter- 
races. 
As already pointed out the amplitude of dune or wind 
ripple-mark shows only slight variation. Subaqueous current 
