H. A. Baker — Loose Arenaceous Sediments. 463 



On the Investigation of the Mechanical Constitution 

 of Loose Arenaceous Sediments by the method of 

 Elutriation, with special reference to the Thanet 

 Beds of the Southern Side of the London Basin. 



By Herbert Arthur Baker, M.Sc, F.G.S. 

 {Concluded from p. 420.) 



Velocities of the Currents from which the Thanet Beds 



WERE deposited. 



THE speeds of tlie currents employed in the elutriation process 

 in sorting diversely sized material bear no relation whatever to 

 the speeds of the currents, which are competent to transport the 

 same material. The elutriation process in no way imitates the natural 

 processes whereby sediments are transported, sorted, and deposited. 

 The determination of competent velocities in relation to size of 

 material transported is quite a distinct investigation, and one which 

 still calls for much study in spite of the time and attention given to 

 it by numerous workers. 



A conclusion which appears to be very generally held is that the 

 diameter of the largest particle a current can move is proportional 

 to the square of the velocity. This is the conclusion to which the 

 theory of impact and transport by a current leads, and results 

 obtained by a large number of investigators appear to endorse it. 

 The law may be stated in the form : — 



d = kV/ 

 where d = diameter of largest particle transported, ' 



^ = a constant, 

 and Fj = the speed of the current which is just competent to 

 transport the particle. 



Difficulty arises, however, as soon as we endeavour to assign 

 a definite value to the constant k. On this point the writer 

 examined the experimental results of many investigators, and found 

 no approach to agreement as to the value of this alleged constant 

 factor. It appears that the value to be assigned to k varies with 

 the class of phenomena observed, and is affected by such considera- 

 tions as whether the particle under transport is to be regarded as 

 (a) an isolated body being transported over a bed of similar 

 material, (6) an isolated body being transported over a bed of 

 different material, either finer or coarser, or (c) an individual particle 

 in a mass of similar material, the whole of which is in process of 

 transport. 



In endeavouring to obtain some idea of the speeds of the currents 

 from which the Thanet Beds Avere deposited, search was first made 

 for data which would afford a value for the factor k in the case 

 of phenomena of type (c) above ; but, although a considerable 

 amount of experimental work on the speeds of currents competent 

 to transport sands and gravels of varying degrees of coarseness has 



