H. A. Baker — Loose Arenaceous Sediments. 



365 



or coarseness of grain. The following selected pair of sediments will 

 suffice to illustrate this point : — 



Both samples are of the Thanet Sand from Messrs. Tuff & Hoar's 

 Pit, East Wickham, but whereas sample A is from the extreme top 

 of the deposit, sample B is from about the middle of the deposit. 

 The mechanical analysis reveals at once that sample ^ is a distinctly 

 coarser sediment than sample B. In the case of sample A, over 

 50 per cent of the material is sand between the limits of '125 and 

 •15 mm., whereas in the case of sample B over 50 per cent is of the 

 coarse silt grade. When the elutriation curves of the two sediments 

 are plotted this distinction is very clearly shown. (See Fig. 2.) The 

 calculation of the Purdy fineness factor should therefore produce 

 the larger number in the case of sample B. But this is not the 

 case, viz, : — 



««*8) + (tT25X-2986) 



•1125 

 •2986 



' ^1125 



+ 2-65 



= 15-64. 



In the case of sample B the figures are :- 

 Purdy factor = 



•0078 , •I 154 , •5268 . -2962 



•0490 . ^0048 



•005 ' ^03 ' -075 

 1-56 +3-85 +7-02 

 15^43 



' -1125 ' ^1375 



+ 2-63 +-35 



•175 

 ■02 



Hence we see that the very insignificant difference obtained is on 

 the wrong side. 



It becomes necessary, therefore, to seek another basis of 



