hout considering the needs of the particular road on which it is
to be used, is not unlike an ill person taking the nearest medicine at
hand, without reference to the nature of the malady or the properties of
the drug. If a road is bad, the exact trouble must first be ascertained
before the proper remedy can be applied. If the surface of a macadam
road continues to be too muddy or dusty after the necessary drainage
precautions have been followed, then the rock of which it is constructed
lacks sufficient hardness or toughness to meet the traffic to which it
is subjected. If, on the contrary, the fine binding material of the
surface is carried off by wind and rain and is not replaced by the wear
of the coarser fragments, the surface stones will soon loosen and allow
water to make its way freely to the foundation and bring about the
destruction of the road. Such conditions are brought about by an excess
of hardness or toughness of the rock for the traffic. Under all
conditions a rock of high cementing value is desirable; for, other
things being equal, such a rock better resists the wear of traffic and
the action of wind and rain. This subject, however, will be referred to
again.

Until comparatively recent years but little was known of the relative
values of the different varieties of rock as road material, and good
results were obtained more by chance and general observation than
through any special knowledge of the subject. These conditions, however,
do not obtain at present, for the subject has received a great deal of
careful study, and a fairly accurate estimate can be made of the
fitness of a rock for any conditions of climate and traffic.

In road-building the attempt should be made to get a perfectly smooth
surface, not too hard, too slippery, or too noisy, and as free as
possible from mud and dust, and these results are to be attained and
maintained as cheaply as possible. Such results, however, can only be
had by selecting the material and methods of construction best suited to
the conditions.

In selecting a road material it is well to consider the agencies of
destruction to roads that have to be met. Among the most important are
the wearing action of wheels and horses' feet, frost, rain, and wind. To
find materials that can best withstand these agencies under all
conditions is the great problem that confronts the road-builder.

Before going further, it will be well to consider some of the physical
propertie