ome roads to far beyond the limit of the
economical capacity of this type of pavement on others, it follows
that any particular deposit of stone might be durable enough for some
roads, while for others it might be entirely inadequate. As a general
rule it has been found that stone that wears away at a moderate rate
will, when used for water-bound macadam surface, result in a smoother
trackway than one that will wear very slowly. It is not therefore
altogether certain that the most durable stone to be had should be
selected for a particular road. This is especially true now that the
water-bound macadam surface has been largely superseded for trunk line
highways and other heavily traveled roads, and is employed in
locations where service conditions are not severe.

The stone employed for the water-bound macadam surface must possess
good cementing properties, because the surface depends for stability
primarily upon the bonding action of the dust from the broken stone.
This is in contrast to the gravel road, where little dependence is
placed upon the bonding effect of the rock dust. In preparing the
stone for macadam surfaces, the ledge rock is crushed and screened,
and in that way a supply of the finer particles, which are a part of
the output of the crusher, is obtained for use in bonding the
surface. This finely broken material, usually called screenings, is
essential to the construction of the water-bound type of surface.
Rocks vary considerably in the cementing properties of the dust, but
usually the rocks classed as "trap," such as andesite, gabbro and
rhyolite, and schist and basalt possess good cementing properties.
Limestones usually possess good cementing properties, but some of the
dolomitic limestones are of low cementing value. Quartz, sandstone and
the granites are of low cementing value.

=Kinds of Rocks Used for Macadam.=--Limestone and chert are the two
sedimentary rocks, employed most extensively for broken stone roads.
These rocks are found in widely distributed areas and vary in physical
characteristics from very soft material of no use to the road builder
to materials possessing considerable durability. It is desirable to
carefully test out the deposits of these materials before using to
ascertain the probable value of the rock, for the construction of the
road surface.

Of the igneous rocks, those classed as trap are best known to the road
builder and many of the deposits of trap rock afford an exce