to earthy material mixed with the rock particles, or to
both. Table 7 shows the results of a number of tests made upon gravels
and indicates that the cementing property of the gravel does not
always depend upon the clay content.

=Permissible Size of Pebbles.=--The larger pebbles in the gravel are
less likely to crush under loads than smaller pebbles of the same sort
of rock, but if the rock is of some of the tougher varieties such as
trap, there is very little likelihood of even the smaller pebbles
crushing. If the pebbles are of rock of medium toughness, the smaller
pebbles might be crushed under the heavier loads. It is the usual
practice to permit gravel to be used for the foundation course in
which the pebbles are as large as will pass a 3-1/2-inch circular
screen opening, and for the wearing course, as large as will pass a
2-1/2-inch circular screen opening. If larger pebbles are allowed in
the wearing course, the surface is certain to become rough after a
time. If the gravel is to be placed in a single course as is a very
common practice, then the maximum size should not exceed that which
will pass a 2-1/2-inch circular screen opening.

The Wisconsin Highway Commission has constructed a very large mileage
of excellent gravel roads and the sizes specified for their roads are
as follows:

"_Bottom Course Gravel_.--Bottom course shall consist of a
mixture of gravel, sand and clay with the proportions and various
sizes as follows:

"All to pass a two-inch screen and to have at least sixty and not
more than seventy-five per cent retained on a quarter-inch
screen; at least twenty-five and not more than seventy-five per
cent of the total coarse aggregate to be retained on a one-inch
screen; at least sixty-five and not more than eighty-five per
cent of the total fine aggregate to be retained on a two
hundred-mesh sieve."

"_Top Course Gravel_.--Top course shall consist of a mixture of
gravel, sand and clay with the proportions of the various sizes
as follows:

"All to pass a one-inch screen and to have at least fifty and not
more than seventy-five per cent retained on a quarter-inch
screen; at least twenty-five and not more than seventy-five per
cent of the total coarse aggregate (material over one-fourth inch
in size) to be retained on a one-half-inch screen; at least
sixty-five and not more than eighty-five per cent o