﻿ITTJ i'^cb. 



sio i^'^^^- 



0-40 foot. 



0-13 foot. 



0-44 foot. 



0-17 foot. 



Yol. 64.] QUANTITATIVE METHODS TO THE STUDY OF ROCKS. 181 



Assuming, for the sake of simplicity, that the effective action of a 

 current on grains of sand of varying size but of similar shape varies 

 directly as the exposed surface, and that the frictional resistance to 

 be overcome varies directly as their weight, the velocity of a 

 current just able to drift them along would be when these two 

 quantities are equal ; that is to say, roughly speaking, that the 

 necessary motive power would vary directly as the size of the grains. 

 Now, as already described, the mean velocity just able to drift grains 

 a hundredth of an inch in diameter is a little under -5 foot per 

 second, or say '4 foot. Hence, in the case of Millstone-Grit sand 

 measuring a twentieth of an inch in diameter, it would be about 

 2 feet per second ; and for the very fine Alum-Bay sand^ ^^ 

 inch in diameter, it would be -13 foot. Combining these results 

 with those deduced from the angles of rest, we obtain the 

 following : — 



Table III. 



Size of grains -g-^jr inch. 



Meaii current just able to drift ... 2*00 feet. 

 Mean current just able to wash up 2*24 feet. 



Though deduced from entirely different data, these results agree 

 well with the fact that the weight of the grains varies as the cube 

 of their diameter, and the surface exposed to the current varies 

 as the square. 



As shown later, the velocity at the very bottom just enough'to 

 drift the sand up the ripples is only -09 foot, which agrees with the 

 fact that a velocity of 12 inches washes up the sand vigorously; 

 and, when the mean velocity is 18 inches and that at the bottom 3, 

 the sand can scarcely maintain itself. We thus have two important 

 limits — one just sufficient to wash it along, and the other to wash 

 it away, which had no upper limit. 



Y. Ripple-Drift. 



Only that structure is considered which is produced by a current 

 moving in one direction, as shown by the detailed characters. This 

 is a most interesting structure, since it enables us to ascertain with 

 approximate accuracy, not only the direction of the current and 

 its velocity in feet per second, but also the rate of deposition in 

 fractions of an inch per minute. This introduction of minutes and 

 seconds into geology may probably surprise those who are accus- 

 tomed to deal with long geological periods, but it must be remem- 

 bered that my minutes and seconds can be verified by experiment, 

 which cannot be done with their long periods. 



The production of this structure (see Pis. XY k XYI) involves a 

 number of variable conditions, namely, the depth and velocity of the 

 current ; the size, shape, and density of the drifted material ; the 

 length and height of the ripples, and the rate at which deposition 



