322 



-Loose Arenaceous Sediments. 



. Gravel grade. 

 Very coarse sand ' 



Coarse sand 



Medium sand 



Fine sand 



Sand grade. 



Coarse silt 



- Silt grade. 

 Fine silt 



. Clay grade. 



Deposits consisting wholly of material of one or other of the 

 grades specified above do not ajopear to occur in nature. Detrital 

 sediments consist of a mixture of certain proportions by weight of 

 many of the above grades, and it is the purpose of a mechanical 

 analysis to determine the proportionate weights of the grades 

 present in the sediment under examination. The allocation of a 

 descriptive name to a sediment depends, usually, upon the presence 

 in it of some one particular grade which occurs in sufficiently marked 

 preponderance to influence the general character of the whole. But 

 proportionality of the various grades is often of great importance 

 in influencing the nature of a sediment, especially in the case of the 

 finer arenaceous deposits. A sandy sediment may contain a per- 

 centage-weight of the clay grade, which, though small, is nevertheless 

 enough to bring up the capillarity and water-capacity of the whole 

 to a value sufficient to result in considerable binding-power. Such 

 a sediment, when seen wet, in the field, would be liable to be termed 

 " clay ", although the same material, seen dry, in the laboratory, 

 would probably be termed " sand ". These points emj)hasize the 

 need for a definite classification of sediments, based fundamentally 

 upon the grades present and the pro|)ortionality by weight of these 

 grades. 



Early in 1919 the opportunity was afforded the present writer 

 of undertaking, at the Imperial College of Science and Technology, 

 a piece of work which he had long had in mind, namely, a study of 

 the Lower London Tertiary strata of the southern side of the London 

 Basin. The sediments to be dealt with being in the main arenaceous, 

 it appeared likely that useful results would be afforded by detailed 

 mechanical analyses. Professor P. G. H. Boswell had foimd such 

 analytical data of much service in his study of various deposits 

 (including the Lower London Tertiaries) in East Anglia, and had 

 further shown the great value of such work upon sediments from the 

 economic point of view. Accordingly much time was spent in 

 carrying out a very large number of careful mechanical analyses, 

 and the opportunity was taken of revising the methods of experiment 

 in this connexion. From results obtained, the writer is convinced 

 of the usefulness and importance of this line of work to geologists, 



