By the mechanical composition of a sediment we understand the proportions of particles of different specified sizes it contains. Stated in other terms, the mechanical composition of a sediment is the frequency distribution in it of particles of different specified sizes, the frequency being measured by the weight of the particles, not by their numbers. In most natural sediments particles of about a certain specific size predominate—that is, their number is greater than that of larger or smaller particles. In some sediments this predominant or mean size represents what statisticians would call a pronounced central tendency. In certain respects the bell-shaped curve representing the frequency distribution or central tendency in the size of the particles, like many other similar curves representing physical or biological data, is closely related to the probability curve, or the so-called normal curve of error.2

In many sediments the deviation in the size of . . .

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