s
for road-building. The tougher limestones are very good, but the softer
ones, though they bind and make a smooth surface very quickly, are too
weak for heavy loads; they wear, wash, and blow away very rapidly.

The materials employed for surfacing roads should be both hard and
tough, and should possess by all means cementing and recementing
qualities. For the Southern States, where there are no frosts to contend
with, the best qualities of limestone are considered quite satisfactory
so far as the cementing and recementing qualities are concerned; but in
most cases roads of this class of material do not stand the wear and
tear of traffic like those built of trap rock, and when exposed to the
severe northern winters such material disintegrates very rapidly. In
fact, trap rock, "nigger heads," technically known as diabase, and
diorites, are considered by most road engineers of long experience to be
the very best stones for road-building. Trap rocks as a rule possess all
the qualities most desired for road stones. They are hard and tough, and
when properly broken to small sizes and rolled thoroughly, cement and
consolidate into a smooth, hard crust which is impervious to water, and
the broken particles are so heavy that they are not readily broken or
washed away.

Unfortunately the most useful stones for road-building are the most
difficult to prepare, and as trap rocks are harder to break than any
other stones they usually cost more. The foundation or lower courses may
be formed of some of the softer stones like gneiss or limestone, but
trap rock should be used for the wearing surface, if possible, even if
it has to be brought from a distance.

As to the construction of macadam roads, Mr. Potter says:

"In the construction of a macadam road in any given locality, the
question of economy generally compels us to use a material found near at
hand, and where a local quarry does not exist field stone and stone
gathered from the beds of rivers and small streams may often be made to
serve every purpose. Many of the stones and boulders thus obtained are
of trap rock, and in general it may be said that all hard field and
river stones, if broken to a proper size, will make fairly good and
sometimes very excellent road metal. No elaborate test is required to
determine the hardness of any given specimen. A steel hammer in the
hands of an intelligent workman will reveal in a general way the
relative degree of toughness of two or