4:5V' 



97-69 



H. A. Baker — Loose Arenaceous Sediments. 465 



total volume found by displacement. The diameter of the average 

 pebble, assumed spherical, was then calculated. The result was 

 2' 197 cm. The average specific gravity of the flint was then 

 determined (2"59) and the weight in lb. of a cubic foot of flint 

 calculated (16r69). These figures were then applied to Owens' 

 formula thus : — 



•02197 X 39-3708 



giving F = 1-37 ft./sec. 



This value may be taken as the minimum speed (probably not much 

 below the true speed) of the current from which the sand associated 

 with the pebbles was deposited. A minute mechanical analysis of 

 the sand was next made, the elutriation-curve drawn, and the 

 Equivalent Grade figure determined with great precision (•102 mm.). 

 The sand proved, fortunately, to give a curve of remarkable 

 similarity to a Thanet Sand of intermediate type, with a fairly high 

 Grading Factor. The Equivalent Grade of the sand was taken as 

 the representative diameter of the material. It was a figure on 

 which much reliance could be placed, in view of the high Grading 

 Factor. Then, by substitution in the formula 



d = lV,' 

 the value to be assigned to the factor k (for transport of sand in 

 bulk) was ascertained, thus : — 



-102 = kx (1-37)2 

 d being taken in mm. and F in feet per second. 



Hence k^^=^ -054. 



The value of k, as obtained directly from Owens' formula (by 

 substituting in it the weight in lb. of a cubic foot of quartz), is of 

 a much larger order than that obtained above, so that velocities 

 calculated by means of it are much less than those obtained when 

 taking the value indicated above. It has long been known, however, 

 that an aggregate requires higher velocity to move it than does a 

 single object. T. E. Blackwell demonstrated this in 1857, and Owens 

 himself noted that the velocities required to move sand-grains in 

 the mass were several times greater than the velocities as calculated 

 from his formula, on substituting in it the diameter of the 

 individual grains. In view of the use to which the above 

 result was to be put, the writer thought it well to check 

 the formula by repeating the experiment with a fresh sample 

 of sand containing flint pebbles obtained from another area. 

 A second sample was taken, this time from the lower part of 

 the Woolwich and Reading Beds, at a spot many miles distant 

 from the place whence the first sample had been obtained. The 

 result was remarkable. The hundred representative pebbles gave 

 an average diameter very near that obtained in the first experiment 



VOL. LVII. — NO. X. 30 



