s of rock which are important in road-building, for the value
of a road material is dependent in a large measure on the degree to
which it possesses these properties. There are many such properties that
affect road-building, but only three need be mentioned here. They are
hardness, toughness, and cementing or binding power.

By hardness is meant the power possessed by a rock to resist the wearing
action caused by the abrasion of wheels and horses' feet. Toughness, as
understood by road-builders, is the adhesion between the crystal and
fine particles of a rock, which gives it power to resist fracture when
subjected to the blows of traffic. This important property, while
distinct from hardness, is yet intimately associated with it, and can in
a measure make up for a deficiency in hardness. Hardness, for instance,
would be the resistance offered by a rock to the grinding of an emery
wheel; toughness, the resistance to fracture when struck with a hammer.
Cementing or binding power is the property possessed by the dust of a
rock to act, after wetting, as a cement to the coarser fragments
composing the road, binding them together and forming a smooth,
impervious shell over the surface. Such a shell, formed by a rock of
high cementing value, protects the underlying material from wear and
acts as a cushion to the blows from horses' feet, and at the same time
resists the waste of material caused by wind and rain, and preserves the
foundation by shedding the surface water. Binding power is thus,
probably, the most important property to be sought for in a
road-building rock, as its presence is always necessary for the best
results. The hardness and toughness of the binder surface more than of
the rock itself represents the hardness and toughness of the road, for
if the weight of traffic is sufficient to destroy the bond of
cementation of the surface, the stones below are soon loosened and
forced out of place. When there is an absence of binding material, which
often occurs when the rock is too hard for the traffic to which it is
subjected, the road soon loosens or ravels.

Experience shows that a rock possessing all three of the properties
mentioned in a high degree does not under all conditions make a good
road material; on the contrary, under certain conditions it may be
altogether unsuitable. As an illustration of this, if a country road or
city park way, where only a light traffic prevails, were built of a
very hard and tough