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

Extract

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 0 to 100, and upwards, along a vertical ordinate, a scale of grade-size limits from 0 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, intercepted 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.)