108 P.G.H. Boswell—Quantitative Methods in Stratigraphy. 
obtained, if required, by treatment with circular-holed sieves, but 
separation below °5 mm. by sifting is hardly satisfactory. 
A more detailed classification, such as the following, has then been 
found useful, and may be carried out with adaptations of the Schoene 
apparatus. 
Diameter in mm. 
Fine gravel , : Shes 
Coarse sand : ; 4.) = fomands<" 
Medium sand . F . >:265and<-5 >} Sand grade. 
Fine sand . > Levande < 25 
Superfine sand (or coarse silt) > 05 and <1 ie 
ae silt ( : : > 01 and < -05 geo 
Clay : : : A < O01 Mud grade. 
The mud grade may be divided, if desired, into portions of diameter 
greater and less than -005 mm. 
In sediments the portion of diameter between 1 mm. and ‘01 mm. 
is found to be most amenable to treatment for heavy residues; that 
over 1 mm. diameter frequently contains compound grains, and consists. 
of the lighter minerals, while that below -01 mm. diameter, being 
a mud grade, tends to clog the heavy liquids during separation. 
During deposition (as during elutriation—the reverse “process) the 
final velocities attained by small grains are dependent upon their 
surface areas (and therefore their diameters) and shape, rather than 
upon their densities. Nevertheless, as a result of greater density, it 
is found that the heavy detrital grains in any sediment often have an 
average diameter less than that of the lighter constituents. Minerals 
like mica, which have an excellent cleavage and therefore tend to 
occur in thin plates with a large flat surface, settle down slowly, and 
have, in spite of their greater density, a greater diameter than that 
of the accompanying quartz and felspar. Probably a constant ratio 
exists between the surface area (and also volume, for the thickness. 
probably varies with the diameter) and that of the other grains, both 
of light and heavy minerals. Asa result of measurement of material 
from various British sedimentary rocks, it is concluded that the 
volume of each muscovite grain is usually rather less than that of 
the average grain of quartz and felspar (perhaps about 80 per cent), 
but that the diameter of the flakes varies from two to four times that 
of the other heavy minerals. (The thickness of the mica flakes has 
been measured by focussing methods and birefringence.) The following 
are a few actual examples, selected from a large number :— 
Diameter in mm. 
Other heavy detrital 
Muscovite. minerals. 
Bunter pebble-bed, Devon (Dr. H. H. ag 5 2 to -3 
Yeovil Sands F 25 -06 
Lower Greensand, Hunstanton: : : : 6 25 
Thanet Beds, Bramford F : : : 15 04 
London Clay, Holbrook : : é : “2 -08 
Claygate Beds : : : : : 15 -05 
Boxstones (Crag) . : . 2 : : -5 to +6 2 
Lenham Beds ‘ j , : : : “4 2 
Elutriation methods depend upon the final velocities of subsidence 
