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KIRTLEY r. MATHEK 
actuallj^ retreating, not so much by sea erosion as by settling, 
which in places predominates over upbuilding.” This com- 
pacting of material must necessarily squeeze out fluids from 
the dwindling pores. 
Sedimentary debris comprises three classes of material, clay 
particles, sand grains, and calcareous fragments, distinguished 
on the basis of composition and shape of individual particles 
rather than on the basis of size. Theoretically there should 
be an equal amount of compacting of earthy materials of the 
same composition and individual shape, under the same weight 
of overburden and with an equal time for adjustment, regardless 
of the dimensions of the the particles. Such classifications ac- 
cording to size as that of the Bureau of Soils, which applies the 
term sand to all earthy materials with dimensions between 0.05 
and 1.0 mm., the term silt to those between 0.005 and 0.05 mm., 
and the term clay to all particles less than 0.005 mm. in diameter, 
are of value in this connection merely because they are also 
more or less perfect classifications according to chemical and 
mineral composition and therefore according to shape. Silt 
and clay particles are in a large degree tabular in shape, so they 
may fit together, like bricks in a wall, with small and few voids 
between them. Resistance to crushing of individual flakes is 
comparatively slight, so that flakes which do not assume ap- 
proximately parallel orientation are broken and the fragments 
pushed into the proper plane. Sand grains, on the contrary, 
are roughly equidimensional or spheroidal in shape, so that they 
must fit together, like field stones in rubble-work, with large 
interstices between them. Crushing strength of individual 
grains is proportionally very much greater. Consequently, 
in any bedded deposit of interstratified clay and sand, the com- 
pacting of the clay strata is many times as great as that of the 
sand layers, and because reduction of pore space is propor- 
tional to amount of compacting, fluids will be squeezed from the 
midst of the clay much more completely than from the voids in 
the sandy beds. Calcareous debris would probably be inter- 
mediate between clay and sand in its reaction to compacting 
stresses. 
