[Illustration: OYSTER-SHELL OBJECT-LESSON ROAD

[_In course of construction, near Mobile, Alabama_]]

The practice of using too soft, too brittle, or rotten material on roads
cannot be too severely condemned. Some people seem to think that if a
stone quarries easily, breaks easily, and packs readily, it is the
very best stone for road-building. This practice, together with that of
placing the material on unimproved foundations and leaving it thus for
traffic to consolidate, has done a great deal to destroy the confidence
of many people in stone roads. There is no reason in the world why a
road should not last for ages if it is built of good material and kept
in proper repair. If this is not done, the money spent is more than
wasted. It is more economical, as a rule, to bring good materials a long
distance by rail or water than to employ inferior ones procured close at
hand.

The durability of roads depends largely upon the power of the materials
of which they are composed to resist those natural and artificial forces
which are constantly acting to destroy them. The fragments of which they
are constructed are liable to be attacked in cold climates by frost, and
in all climates by water and wind. If composed of stone or gravel, the
particles are constantly grinding against each other and being exposed
to the impact of the tires of vehicles and the feet of animals.
Atmospheric agencies are also at work decomposing and disintegrating
the material. It is obviously necessary, therefore, that great care be
exercised in selecting for the surfacing of roads those stones which are
less liable to be destroyed or decomposed by these physical, dynamical,
and chemical forces.

Siliceous materials, those composed of flint or quartz, although hard,
are brittle and deficient in toughness. Granite is not desirable because
it is composed of three materials of different natures, viz., quartz,
feldspar, and mica, the first of which is brittle, the second liable to
decompose rapidly, and the third laminable or of a scaly or layerlike
nature. Some granites which contain hornblende instead of feldspar are
desirable. The darker the variety the better. Gneiss, which is composed
of quartz, feldspar, and mica, more or less distinctly slaty, is
inferior to granite. Mica-slate stones are altogether useless. The
argillaceous slates or clayey slates make a smooth surface, but one
which is easily destroyed when wet. The sandstones are utterly useles