H. A. Baker — Loose Arenaceous Sediments. 863 



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. 



{Continued from y. 832.) 



Graphical Expression of Mechanical Analyses. 



IN giving graphical expression to the data obtained by mechanical 

 analyses of sediments, the use of curves was found to be very 

 satisfactory. After some consideration the method finally adopted 

 was to mark off along a horizontal abscissa a scale of percentage- 

 weights, from to 100, and upwards, along a vertical ordinate, 

 a scale of grade-size limits from to "2 mm. and beyond. In cases 

 where the range of the diameters of the grains in a sample is very 

 great it would be necessary to employ a method of reducing the 

 representative lengths, and this has sometimes been done by marking 

 off lengths proportional to the logarithms of the diameters ; but 

 in the case of the arenaceous sediments dealt with by the writer, 

 the necessity for this did not arise, and in the curves illustrating 

 this paper, the ordinate lengths shown are directly proportional to 

 the diameters represented. The length along an abscissa, inter- 

 cepted between the first ordinate and the curve, represents the 

 total percentage weight of the sample which, up to the limit of size 

 specified, has been washed off, and the remaining length signifies 

 the percentage-weight yet remaining to be dealt with. A tendency 

 towards verticality on the part of the curve denotes relative absence 

 or paucity of material of the sizes specified, and, correspondingly, 

 a tendency towards horizontality signifies a preponderance of 

 material of the size specified. Hence, a fine-grained sediment 

 will give a curve occupying the lower part of the diagram, and 

 with increasing coarseness of sediment the curve climbs successively 

 higher and higher. (See Figs. 2, 3, and 6.) 



Treatment of Data yielded by Mechanical Analyses. 

 (1) Failure of the " Purdy Fineness Factor ". 

 A method whereby the mechanical constitution of a sediment 

 can be expressed by means of a representative number has been 

 much sought after, but up to the present no really satisfactory 

 result has been achieved. Such a number, could it be obtained, 

 would undoubtedly be of great utility. It could be plotted upon 

 maps, and ideas connecting the mechanical constitution of a sediment 

 with its geographical distribution could be gained. The need for 

 such a number in connexion with the economic aspect of the 

 mechanical composition of sands and clays is much felt. In 

 America an attempt has been made to obtain a " fineness factor " 

 which shall be proportional to the total surface-area of the sample. 



