26 
MUSEUM BULLETIN NO. 25. 
time there results a regular pattern of pareliel ridges, equally 
spaced. It has been found experimentally that by varying the 
amplitude of the water oscillation and also by varying its fre- 
quency the size of the resulting ripples can be controlled ; but the 
precise laws of control have not been demonstrated. Evidently 
the frequency of the natural oscillation equals the frequency of 
the wind waves, and its amplitude is a function of the size of the 
waves and the depth of the water; so that a relation will ulti- 
mately be established between wave-size, wave-period and water- 
depth as conditions and ripple-size as a result.” 1 
The reasons for the initiation of the ripple-mark ridges 
have been discussed in some detail by Hertha Ayrton. 2 * 
Although the symmetrical or oscillation ripple-mark is the 
prevailing type in lake sediments, asymmetric ripples are 
occasionally developed on lake bottoms by currents set up by 
storms or other causes. De la Bache® as early as 1851 clearly 
distinguished the wave from the current type of ripple mark, and 
Prof. F. A. Forrell 4 * pointed out many years ago how ripple-marks 
made by wave action differ in form from those made by river and 
tidal currents. Prof. Barrell 6 has also noted the marked differ- 
ence in symmetry between wave and current formed ripple- 
mark. But geologists appear generally to have failed to 
appreciate the importance of the distinction, and have made no 
use of the important clue which the determination of the type 
of ripple-mark may give concerning the history of a ripple-marked 
formation. 
The symmetrical or oscillation type of ripple-mark is less 
accessible to study than the current ripple-mark. It is the pre- 
vailing type in lake waters where the bottom is sandy and where, 
owing to the absence of tides, it is seldom uncovered by the water. 
Examination of symmetrical ripples is, therefore, restricted 
ordinarily to viewing them through water of varying depths. 
1 Gilbert, G. K., “Ripple-marka and crosa-bedding," Bull. Geol. Soc. Am., vol. X, 1899, 
pp. 137-138, 
* "The Origin and growth of ripple-mark," Proc. Roy. Soc., London, vol. 84, 1911, pp, 
285-310, 
* The Geological observer, 1851, pp, 607-dl0. 
* Bull, de la SocMtfe Vandoise des Sciences Naturelles, Feb. 1878. 
* Bull. Geol. Soc. Am., rol. XXIII. 1912, p. 432. 
