Page 15 - Petrology of Sedimentary Rocks

which reduces to
w l

P

COMPACT

PLATY 33 BLADE0 6’ ELONGATE

Form triangle. Shapes of particles falling at various points on the triangle are illustrated by a
series of blocks with axes of the correct ratio; all blocks have the same volume. Independence of the concepts
of sphericity and form may be demonstrated by following an isosphericity contour from the disklike extreme
at the left to the rodlike extreme at the right.

For two-dimensional purposes (as in thin sections) two other “sphericity” measure-
ments have been applied. Riley Sphericity is given as

II-

* Di

D,

where DC is the diameter of the smallest circumscribing circle and Di is the diameter
of the largest inscribed circle. These can be easily measured by a celluloid scale ruled
off in a series of closely-spaced concentric circles of known diameter, which can then
be placed over the sand grain image. Another measure is Elongation which is simply
width (actually least projection width) over length measured by rectangular grid. This
is probably the most satisfactory two dimensional measure (Griffiths; Dapples and
Romi nger).

Although individual grains may have widely varying W/L values, sample means
(obtained by counting 100 quartz grains in one thin section, for example) show a much
more restricted range. Measurement of many sandstones has suggested the following

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