S68 H. A. Baker — Loose Arenaceous Sediments. 



which is done by finding the area under the curve by means of the 

 planimeter and dividing the result by the length representing 

 100 per cent weight. The length so obtained is then interpreted in 

 terms of the scale of lengths employed to represent diameters. 

 The value of this mean ordinate may then very well be taken as the 

 representative diameter, for comparative purposes, of the particles 

 in the sediment. The figure obtained may be termed the 

 " Equivalent Grade " for the particular sediment. 



Fine sediments will give a low figure for the " Equivalent Grade " 

 and the figure will increase with increasing coarseness of sediment. 



On applying this method to the two sediments considered above, 

 when referring to the Purdy factor, we find that sample A (the 

 coarser sediment) gives an Equivalent Grade of •120 mm., whereas 

 the finer sample B gives "087 mm. As the result of applying the 

 method to the data obtained by carrying out a very large number 

 of mechanical analyses of arenaceous sediments, the writer now 

 concludes that we have, in this " Equivalent Grade ", a valuable 

 means of comparing sediments from the point of view of their 

 coarseness or fineness of grain, and one superior to any method 

 at present in vogue. 



(3) The " Grading Factor" of a Sediment. 



It cannot be said, however, that the Equivalent Grade supplies 

 a figure which is unique for each and every sediment. The elutriation 

 curve of a sediment is unique — each and every sediment gives its 

 own particular curve — and only two mechanically identical sediments 

 would yield identical curves. It is possible, however, for two different 

 curves to yield the same Equivalent Grade figure. Hence it is 

 necessary for a further factor to be obtained, based upon the 

 respective j)roportionalities of the grades present in the sediment, 

 so that this second figure, taken in conjunction with the Equivalent 

 Grade, affords a pair of numbers which together give complete 

 uniqueness to any sediment. The writer has devised a means 

 whereby a second figure — a " Grading Factor " — can be obtained, 

 so that if two sediments give the same Equivalent Grade they differ 

 in their Grading Factor, and vice versa. Incidentally, the 

 Equivalent Grade and the Grading Factor of a sediment are figures 

 which should prove of the utmost utility in connexion with the 

 economic aspect of the question of the mechanical constitution of 

 sediments, and, further, there is every reason to suppose that as 

 our knowledge of the conditions governing the transport, sorting, 

 and deposition of sediments progresses, they will, in addition, 

 prove of considerable value to the geologist. 



The Grading Factor of a sediment is found as follows : Curve C 

 in Fig. 2 represents the elutriation-curve of a sediment (Lower 

 Thanet Beds, Charlton Pit), which gave the following mechanical 

 analysis : — 



