tl. A. Baker — Loose Arenaceous Sedi'rnents. 



369 



The Equivalent Grade of this sediment, determined from the 

 curve, is 'O-SO mm., and its equivalent, perfectly graded ideal sediment 

 is represented by the horizontal abscissa inserted on the graph in 

 the appropriate place. 



Now, the area enclosed between the first ordinate, the Equivalent 

 Grade line, and the curve, represents the measure of the variation 

 of the sediment (having regard to diameters and percentage-weights) 

 below the grade of the ideal, perfectly graded equivalent sediment, 

 and the corresponding equal area enclosed between the last 

 ordinate, the Equivalent Grade line, and the curve, similarly repre- 

 sents the measure of the variation of the sediment above the grade 

 of its ideal, perfectly graded equivalent. Hence, the sum of these 

 two areas gives the measure of the total variation of the sediment 

 from the hypothetical p)erfectly graded equivalent. If we now sub- 

 tract this variation-area from the total area under the curve we get 

 a remainder which expresses the measure of the tendency of the 

 sediment towards constancy of grading or restriction of the diameters 

 of the constituent grains to one specified size, namely that of the 

 Equivalent Grade. In the case of a hypothetical perfectly graded 

 sediment, that is, one in which there is perfect constancy of grading, 

 or complete restriction of the grain diameters to one particular 

 dimension, there is no variation-area, consequently the area which 

 expresses the measure of its tendency towards grading perfection 

 is the area under the curve. 



Hence, if we consider the ratio — 

 Area expressing measure of tendency towards grading perfection 



Total area under curve 

 we see that it assumes the value unity when the grading is perfect. 

 The Grading Factors of naturally occurring sediments will approach 

 nearer and nearer to the value unity as their degree of grading 

 approaches nearer and nearer to perfection. Hence, the Grading 

 Factor of a sediment is given by : — 



Total area under curve — Total variation area 

 Total area under curve 



Determining, with the aid of the planimeter, the Grading Factor 

 of the sediment under consideration, we obtain the value "581. 



Shown on the same diagram (curve D) is the elutriation-curve 

 of another sediment (Lower Thanet Beds, Tufi & Hoar's Pit, East 

 Wickham), which gave the following mechanical analysis :• — 



VOL. LVII.— NO. VIII. 24 



