countered in maintaining a surface of bituminous
material and stone chips on this type of road than on the water-bound
macadam. The extent to which these expectations have been realized has
varied to a marked degree and although some excellent surfaces have
been constructed by this method, the results have as a rule been
neither uniform nor entirely satisfactory. It seems to be apparent
that good results cannot be obtained unless the materials are entirely
suitable and the construction is carried out with unusual skill.

=Foundation.=--The foundation or lower course consists of a layer of
broken stone six inches thick placed on a well drained and thoroughly
rolled earth subgrade. In exceptional cases, the Telford type of
foundation might be employed.

The lower course of broken stone is finished in the same manner as
water-bound macadam, being bonded with stone screenings or with fine
gravel of high clay content.

Since this course is in reality the foundation of the surface, it is
necessary to secure stability by appropriate construction methods,
exactly as in constructing water-bound macadam.

[Illustration: Fig. 21.--Type of Roller used on Gravel and Macadam
Roads]

=Upper or Wearing Course.=--The wearing course consists of a layer of
stone about two and one-half inches thick. The stone is placed and
rolled and the spaces between the stones partially filled with some
suitable bituminous material. The bituminous material is usually
applied by means of a mechanical spreading device connected to a tank
wagon. The bituminous materials employed for this class of
construction are semi-solid in character and must be heated to give
them sufficient fluidity for application. They may be heated in the
tank wagon which is used for the application or they may be heated in
separate tanks and transferred to the distributing wagon for
spreading. Some kind of a nozzle or group of nozzles is employed for
spreading the material so that it can be delivered in the form of a
spray or at least in a thin fan-shaped stream and can be distributed
in a fairly uniform layer over the stone. The binder will cool rather
rapidly after it is applied, but meanwhile will flow into the openings
between the stones and will form over the surface stones a coating of
slight thickness.

The surface of the macadam is next covered with a layer of chips of
tough rock, similar to the material used for the final dressing in
surface treatments. These are