rock with a high cementing value, neither the best,
nor, if a softer rock were available, would the cheapest results be
obtained. Such a rock would so effectively resist the wear of a light
traffic that the amount of fine dust worn off would be carried away by
wind and rain faster than it would be supplied by wear. Consequently the
binder supplied by wear would be insufficient, and if not supplied from
some other source the road would soon go to pieces. The first cost of
such a rock would in most instances be greater than that of a softer one
and the necessary repairs resulting from its use would also be very
expensive.

A very good illustration of this point is the first road built by the
Massachusetts Highway Commission. This road is on the island of
Nantucket and was subjected to a very light traffic. The commission
desired to build the best possible road, and consequently ordered a very
hard and tough trap rock from Salem, considered then to be the best
macadam rock in the state. Delivered on the road this rock cost $3.50
per ton, the excessive price being due to the cost of transportation.
The road was in every way properly constructed, and thoroughly rolled
with a steam roller; but in spite of every precaution it soon began to
ravel, and repeated rolling was only of temporary benefit, for the rock
was too hard and tough for the traffic. Subsequently, when the road was
resurfaced with limestone, which was much softer than the trap, it
became excellent. Since then all roads built on the island have been
constructed of native granite bowlders with good results, and at a much
lower cost.

If, however, this hard and tough rock, which gave such poor results at
Nantucket, were used on a road where the traffic was sufficient to wear
off an ample supply of binder, very much better results would be
obtained than if a rock lacking both hardness and toughness were used;
for, in the latter case, the wear would be so great that ruts would be
formed which would prevent rain water draining from the surface. The
water thus collecting on the surface would soon make its way to the
foundation and destroy the road. The dust in dry weather would also be
excessive.

Only two examples of the misuse of a road material have been given, but,
as they represent extreme conditions, it is easy to see the large number
of intermediate mistakes that can be made, for there are few rocks even
of the same variety that possess the same physical pr