H. A. Baker — Loose Arenaceous Sediments. 



415 



Fig. 4 indicates the comparative instability of conditions of sedi- 

 mentation in West Kent during the deposition of the Thanet Beds. 

 The diagram (Fig. 5) shows the rough, provisional classification of 

 arenaceous sediments ba,sed upon values of Equivalent Grade and 

 Grading Factor, in use by the writer. The value of the Equivalent 

 Grade factor decides whether a sediment shall be classified as a 

 sand, a silt, or a clay, the limit between sands and silts being taken 

 at "08 mm., and that between silts and clays at '02 mm. The value 

 of the Grading Factor decides whether a sediment shall be classified 

 as a true sand or silt, or as a loam, since the characteristic of the 

 loams is the low value of the Grading Factor. The limiting value 

 of the Grading Factor which separates sands and silts on the one 

 hand from loams on the other, is taken at "6. The classification 

 seems to answer very well. 



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N 



^r^ 



•OS ^ 





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Fig. 5. — Provisional classification of the finer arenaceous sediments, based on 

 Equivalent Grade and Grading Factor values. 



Fig. 6 shows types of elutriation-curves yielded by the Thanet 

 Beds of various localities along the southern side of the London 

 Basin, and the table at the end of the paper gives a selection from a 

 large number of mechanical analyses obtained by the writer. 



The construction of a graph showing the relation between the 

 Grading Factor and Equivalent Grade yields results of remarkable 

 interest in the case of the Thanet Beds. When a series of field 

 samples are taken from successive levels in the Thanet Beds at any 

 one locality and mechanical analyses are carried out and the related 

 values of Grading Factor and Equivalent Grade are plotted on 

 a graph, it is found that the points obtained show a strong tendency 

 to lie on a straight line. What is even more remarkable, however, 



