H. A. Baker — Loose Arenaceous Sedmients. 323 



and hoj)es that it will receive the attention which it deserves at the 

 hands of future workers. 



It is proposed in the present paper to deal with some points 

 connected with the revised methods of experiment in carrying out 

 mechanical analyses, and to give, in outline, the main results arising 

 from this side of the writer's investigations, so far as the Thanet 

 Beds alone are concerned. 



All the data were obtained by elutriating samples of the various 

 sediments in glass tubes of a special shape, by means of upward 

 currents of water at known speeds. Sifting, by means of meshes 

 and sieves, was not resorted to for the following reasons : (1) The 

 data obtained by the use of sieves are not truly comparable with 

 results given by the employment of currents of water, owing to the 

 different behaviour of the individual grains in the two cases. A 

 grain may be considered as possessing three dimensions at right 

 angles to. each other, a maximum, a minimum, and an intermediate 

 dimension. When placed on a sieve it is the intermediate dimension 

 which decides whether the grain pass the sieve or not, since, 

 if this permit, the grain will turn up on end and drop through the 

 sieve. When buoyed up by a current of water the grain turns so 

 as to expose its greatest cross-sectional area to the current, and 

 consequently, in this case, the maximum dimension, as well as the 

 intermediate one, is of large account in deciding whether the grain 

 shall remain in the tube or be washed off. (2) Meshes and sieves, 

 especially fine ones, are liable to be inaccurate. It is not easy to 

 obtain a sieve of any specified gauge, and the apertures in any sieve, 

 especially after a little wear and tear, show a serious variation 

 among themselves. (3) Sieves are very liable to become clogged 

 up. This frequently occurs during a sifting process, so that even 

 after prolonged agitation of the sieves large amounts of fine material 

 will yet remain entangled within the meshes of even coarse sieves. 



Elutriation by Water Currents i^ (1) Physical Basis of the 



Method. 



When mineral grains are allowed to settle freely in a liquid they 

 do so under the influence of gravity, but the rate of subsidence 



^ General accounts of the method of conducting; elutriation experiments have 

 been given by Crook and also byBoswell. For these and for other information 

 concerning the process, consult the following: — 



(1) Crook, Appendix to Hatch &Eastall, Sedimentary Bocks, London. 1913, 

 p. 349. 



(2) Boswell, British Besources of Sands and Bocks used in Glass-making , 

 2nd (complete) ed., London, Longmans, Green & Co.. 1918. 



(3) Boswell, British Besources of Befr actor y Sands for Furnace and Foundry 

 PuriMses, London, Taylor & Francis, 1918. 



(4) Stadler, " Grading Analyses by Elutriation": Trans. List. Min. and 

 Metall., xii, 1912-13, p. 686. 



(5) Keilhack, Lehrhuch der praktischen Geologie, 1908, 2nd ed., Stuttgart. 



(6) Kies, Clays, their Occurrence, Properties, and Uses, 2nd ed., New York, 

 1908. 



