llent
material for broken stone roads where the severest conditions of
traffic are encountered. The trap rocks are tough and durable and
generally possess excellent cementing properties.

Granite and sandstone are seldom used for water-bound macadam as they
possess poor cementing properties and a binder of some kind must be
added to cement the pieces together. For this purpose clay or the
screenings from some other variety of stone may be utilized.

Some other materials are occasionally employed for the construction of
macadam surfaces. Of these, oyster or marine shells, burnt shale, and
slag are most common.

Shells and slag are of rather low durability but possess good
cementing properties. Shale is a makeshift suitable only for very
light traffic roads.

=Sizes of Stone.=--The stone for the wearing course of a macadam road
should be as large as practicable, because the larger the pieces the
more durable the surface. If the individual stones are too large it is
difficult to secure a smooth surface, and large stones will be readily
loosened by tipping as the wheels roll over them. These considerations
limit the size to a maximum of that which will pass a 2-1/2-inch
screen. Stone of excellent wearing qualities may be somewhat smaller,
but never less than that which will just pass a 1-1/2-inch screen.

For the lower course, the size is not particularly important except
where the earth foundation is such as to require special construction.
It is not uncommon to use the same size of stone for both upper and
lower course and yet in many instances stone up to that which will
just pass a 3-1/2-inch screen is used for the lower course. Stone much
smaller in size may also be used successfully, but if the stone is
broken to a smaller size than is required, unnecessary expense is
incurred.

The bonding material is the finer portion of the product of the
crusher, which is called screenings. This material may be so finely
crushed as to pass a one-fourth inch screen, or may be so coarse as to
just pass a one-half inch screen, but in any case must contain all of
the dust and fine material produced by the crusher.

Where the soil and drainage conditions demand an especially stable
foundation course, the Telford type is used. The Telford foundation
consists of a layer of stones of various dimensions that can be laid
so as to give a thickness of 8 inches. These large stones are placed
by hand and therefore the size requirements