H. A. Baker — Loose Arenaceous Sediments. 321 



On the Investigation of the Mechanical Constitution 

 of Loose Arenaceous Sediments by the method of 

 Elutriation, v*^ith special reference to the Thanet 

 Beds of the Southern Side of the London Basin. 



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



npHE 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 interjDretation, 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. 



With regard to the classification into grades, according to size, 

 of the material which is liable to occur in any one sediment, a scheme 

 which gives good results in practice is in current use. It is as 

 follows : — 



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