﻿184 DR. K. C, SORBT OX THE APPLICATION OF [May I908, 



their relation to ripple-drift. Considering all the complex factors, 

 it is satisfactory to find that the observations can be harmonized 

 quantitatively by a few probable suppositions. 



Ripple-Drift with Deposition from Above. 



The structure of ripple-drift shows that when deposit is formed 



from above, it is accumulated on the protected side of the ripple in 



thin layers at the angle of rest. That this deposition would reduce 



their period admits of no doubt, since the protected side would 



advance more rapidly. As the ripples move forward a portion is 



washed up from the exposed side, and an amount equal to that 



deposited from above is left, and covered up by the next ripple 



advancing from behind. It thus seems to follow that the amount 



drifted forward, independent of the deposition, is the same as when 



there is no deposit from above. The question is, what is the effect 



of the deposit on the rate at which this normal amount is washed 



along ? Since the total to be removed would be greater, it is quite 



possible that it would have a retarding influence and lengthen the 



period of the ripple. At the same time, since, as I have shown, 



the velocity of the current some little distance from the bottom is 



considerably greater than on the actual surface of the sand, the 



subsidence of material would increase the velocity of the current at 



the bottom, and therefore shorten the period of the ripples. All 



these suppositions ought to be verified by experiment ; but, in the 



meantime, it seems to me that we may assume provisionally that the 



above-mentioned two influences may so far compensate one another, 



that they may be neglected. I therefore calculate as follows : — 



The normal area of the section of a ripple perpendicular to the 



3"5 X "72 

 surface in the line of the current is — ^ — = 1*26 inch. The 



area of the material deposited is 3'o d, when d is the thickness of 

 the deposit in inches. Then the period of the ripple would be 

 o 1-26 _ 3 



^^ l-26+3-5c^~ 1-H2-8(^' 



These values, however, must be looked upon as only approximate, 

 but yet most probably of the true order of magnitude, and sufficiently 

 near the truth to warrant the conclusions described later. 



The rate of deposition would be — ^-r- 

 ^ period 



Length of Ripples. 



The exact relation between the size of ripples and the conditions 

 under which they are formed requires further study. If consi- 

 deration were confined to those usually seen in rocks, it would 

 appear most probable that their size depends to a great extent on 

 the character of the sand. The smallest that I have seen are only 

 about three-quarters of an inch long in a very fine-grained sandstone. 



