f medium hardness and
toughness is best. For country road traffic it is best to use a
comparatively soft rock of medium toughness. In all cases high cementing
value should be sought, and especially if the locality is very wet or
windy.

Rocks belonging to the same species and having the same name, such as
traps, granites, quartzites, etc., vary almost as much in different
localities in their physical road-building properties as they do from
rocks of distinct species. This variation is also true of the mineral
composition of rocks of the same species, as well as in the size and
arrangement of their crystals. It is impossible, therefore, to classify
rocks for road-building by simply giving their specific names. It can be
said, however, that certain species of rock possess in common some
road-building properties. For instance, the trap[8] rocks as a class are
hard and tough and usually have binding power, and consequently stand
heavy traffic well; and for this reason they are frequently spoken of as
the best rocks for road-building. This, however, is not always true, for
numerous examples can be shown where trap rock having the above
properties in the highest degree has failed to give good results on
light traffic roads. The reason trap rock has gained so much favor with
road-builders is because a large majority of macadam roads in our
country are built to stand an urban traffic, and the traps stand such a
traffic better than any other single class of rocks. There are, however,
other rocks that will stand an urban traffic perfectly well, and there
are traps that are not sufficiently hard and tough for a suburban or
highway traffic. The granites are generally brittle, and many of them do
not bind well, but there are a great many which when used under proper
conditions make excellent roads. The felsites are usually very hard and
brittle, and many have excellent binding power, some varieties being
suitable for the heaviest macadam traffic. Limestones generally bind
well, are soft, and frequently hygroscopic. Quartzites are almost always
very hard, brittle, and have very low binding power. The slates are
usually soft, brittle, and lack binding power.

The above generalizations are of necessity vague, and for practical
purposes are of little value, since rocks of the same variety occurring
in different localities have very wide ranges of character. It
consequently happens in many cases, particularly where there are a
number o