204 The American Geologist. March, vm 
Lyell,* in a diagram showing the outline of the marks, and De La Beche 
has figured the manner of the forward progress. The latter figure has 
been reproduced by Geikie.t For the relative symmetry of single-cur- 
rent and of oscillation ripple-marks compare th3se figures and that of 
the writer in the January Geologist with the outline of oscillation 
ripple-marks given first by De La Bechei and reproduced by Geikie.^ 
The wind-formed ripple-mark is of the same class as that formed by a 
constant current in water, but the new element imposes important 
modifications. On account of the buoyancy of the water a current of 
given strength is much more effective than a similar current in air, and 
so it produces ridges of greater hight and separation, and of coarser 
material. In rivers, moreover, the direction and velocity of the current 
are constant for long periods of time, and so the structures are not con- 
fused and cross-bedded by shifting currents. Again, the water between 
the sand-grains acts as a cement, and the sand consequently becomes 
so firm that the march of the completely formed ridge is very slow. 
Finally, the water is the carrier of large quantities of sediment, which 
it deposits wherever opportunity offers, and thus the material found 
composing ripple-marks in rivers is not usually that in which the 
ripple-mark series began. The constancy of the current does not 
give rise to those incessant vacillations in position of the vortexes which 
occur in the oscillation ripple-mark, and, at greater intervals, in the 
wind-blown sand; hence the successive layers, particularly if the depo- 
sition be rapid, are ordinarily conformable, except that each new layer 
moves slightly forward, in a direction parallel with that of the current. 
We may have ripple-marks of this class in mixed material by two pro- 
cesses. First, if a strong current produce ripple-marks in homogeneous 
sand these will be high and broad, and coarser material, brought by the 
stream, may lodge in the sheltered places without interfering with the 
structure. It is to be noted that a loose pebble of considerable size 
may be more easily moved by the current than the smooth bed of 
firmly packed sand, so arranged as to present the greatest possible re- 
sistance to the current. Second, if the strength of the vortex be suffi- 
cient to move coarse gravel and even pebbles it is evident that the 
ripple-marking process will go on in mixed material quite as well as in 
sand, which is more uniformly finely divided. The strength of the 
vortex will depend upon the strength of the current. So it is quite 
possible that a ridged structure analogous to ripple-marking may be 
produced in beds which are entirely of pebbles, in swift rivers. 
•'Finally," says the writer of the foregoing communication, "could 
ripple-mark exist at all in a current of sufficient velocity to "sweep along 
pebbles the size of an egg'? A current of less than one-half this veloc- 
ity is quoted by Lyell as sufficient to 'tear up fine gravel.' In such 
'Principles of Geologry, ]>. 34:!. 
The Geological Observer, r- 87 ' 1851) . 
^Text-book of Geology, p. 335. 
iOp. cit., p. 508. 
SOp. cit.. p. 507. 
