RECENT AND FOSSIL RIPPLE-MARK. 
25 
of the breaking wave, which continues up the beach as a thin sheet 
of water loaded with sand picked up during its advance, drops 
this load at the instant when it starts back down the beach slope. 
Wave marks are left only by every third or fourth wave where 
I have observed their formation. This is due to the fact that they 
will not form on sand completely saturated with water. A short 
period, equivalent to that separating the larger waves, suffices 
to dry the sand to the extent of letting the outer fringe of the 
larger waves sink instantly into it, thus leaving a little ridge 
instead of partially carrying back the sand load over a saturated 
surface. Under a very hot sun the interval between successive 
waves might suffice for this. 
Symmetrical ripple-mark under a few inches of nearly still 
lake water is shown in Plate XX B. The sharp contrast between 
the symmetrical or oscillation and the asymmetrical types of 
ripple-mark can be clearly seen by referring to Figures 3 and 4. 
The mechanical theory of the formation of oscillation ripple-marks 
has been developed and the general relation of the essential factors 
involved has been shown by the work of Hunt, Forel, Darwin, and 
others. G. K. Gilbert has formulated the essential elements of 
the theory of their formation in the following passage: 
“The ordinary ripple-mark of beaches and rock faces is 
produced by the to-and-fro motion of the water occasioned by the 
passage of wind waves. During the passage of a wave each 
particle of water near the surface rises, moves forward, descends 
and moves back describing an orbit which is approximately 
circular. The orbital motion is communicated downward, with 
gradually diminishing amplitude. Unless the water is deep the 
orbits below the surface are ellipses, the longer axes being hori- 
zontal and close to the bottom the ellipses are nearly flat, so 
that the water merely swings forward and back. It is in this 
oscillating current, periodically reversed, that the sand-ripples 
are formed. A prominence occasions vortices alternately on its 
two sides, and is thereby developed in a symmetric way with 
equal slopes and a sharp apex. There is a strong tendency 
to produce the mole laterally into a ridge, the space between 
ridges is definitely limited by the interference of vortices and in 
