ALBERT

Description: THE speeds of the 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.

Description: WITH regard to the laws governing the transport, sorting, and deposition of sediments, much has still to be learned, but one fact emerges clearly from the mass of present data. During the process of transport by water the load undergoes selective treatment, or, in other words, is sorted. The late Dr. G. K. Gilbert, in the course of his important work on the transportation of débris by running water, carried out experiments with mixtures of several grades of detrital material and found that before the slope of the stream had been definitely established (i.e. before the attainment of uniformity of conditions), and especially when low velocities were used, the current tended to sort the débris, building deposits with the coarser part and continuing to transport the finer material. This fact has a special bearing on the study of natural sediments. The state of equilibrium, or of perfect uniformity of conditions, is seldom, if ever, completely realized in nature, and even when approximately reached is not, as a rule, of long duration.

Description: The mechanical constitution of a detrital sedimentary deposit is the aggregate result of the operation of a complex suite of physical laws which govern transport of detrital material by, and its deposition from, water in motion. These physical laws have been elaborately investigated by many workers, and while much remains obscure, important conclusions have been achieved. The detailed examination of the mechanical constitution of the sediments of the geological column has been neglected, although a few pioneer workers have obtained notable results. Sediments of economic value have received attention, but in the realm of pure geology workers have been slow to take up the investigation. Accumulating results indicate, however, that geologists would do well to devote attention to this line of research. Distinctive results are being obtained from sediments, which, even though they may not lend themselves at the present time to complete interpretation, from the point of view of reconstruction of the conditions under which the said deposits were formed, nevertheless bid fair to provide the geologist with another weapon in his armoury. Fluviatile deposits have been found to possess a mechanical constitution distinct from that of estuarine sediments, and the latter are again clearly differentiated, mechanically, from purely marine formations. It may be remarked, too, that the nomenclature in current use applied to the loose sedimentary deposits lacks that element of scientific precision so desirable and necessary if proper progress is to be made. Such terms as “sand”, “clayey sand”, “sandy clay”, and “clay” are at present employed by different writers in different senses, and some system of classification which would give definiteness to the meaning of these and similar terms appears to be urgently called for. The only satisfactory basis for such a classification must be a quantitative one, depending upon mechanical analyses of sediments. When a sufficiently comprehensive collection of comparative data is available it should be a simple matter to devise an adequate scheme for the classification of the loose detrital sediments.

Description: IT is common knowledge to all geologists interested in the Lower Eocenes of the southern part of the London Basin that, in the Oldhaven (Blackheath) Pebble Beds, pebbles of a brown quartzite are often to be found on searching amongst the rounded flints of which these beds are so largely composed. When geological parties visit sections exposing the pebble-beds of the Lower London Tertiaries, the search for these quartzites usually constitutes one of the items of the field-work to be done. A considerable number of these pebbles must, by this time, have been collected but, although their occurrence has given rise to a good deal of discussion and speculation, nobody appears, so far, to have undertaken a systematic examination of their characters and petrological affinities. The view has been propounded that the study of these pebbles would throw light on the question of the source of the material composing the Lower London Tertiary beds.

Description: 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.)