—Some of these, if cherty or containing a large percentage of iron, may be used; but as a rule they are quite unfitted for use as a road metal.
Flints.
—These, if tough, make excellent roadways; but unfortunately they are sometimes too brittle for heavy traffic. Surface-picked flints are better than those from a quarry.[29]
Pebbles.
—These are found on sea shores and river beds. They are composed of very various rocks, and are much water-worn and rounded; when broken they sometimes answer very well if mixed with gravel to bind them.
Gravel.
—This, if of a flinty character, and not too much mixed with earthy matter, makes good roads for light traffic, if carefully watched or well rolled during formation. Pit gravel should always be screened through wire screens of 1¹⁄₂ to 1³⁄₄ gauge, and the small can be used for footpaths.
In some places it is difficult to obtain any natural stone for the purposes of road metal; in these cases slag from blast furnaces or ordinary clinkers from furnaces are sometimes used. Oyster shells are used on the roadways near the Gulf coasts[30] and charcoal in Michigan, United States.[31] I have myself made a most excellent roadway with coral on the coast of Jamaica, and no doubt many strange materials have been, and still are, used for this purpose.
“I never mix” is an adage that should be followed by surveyors as regards road metal. Do not mix a soft material with one that is harder for either construction or maintenance of a roadway; the effect is what is known as a “bumpy” road, arising from the fact of the soft stone wearing faster than the hard. The hardest metal should be kept for the top or surface layer of the roadway.
As an instance of the extreme difficulty besetting the question of the best material for road metal, I will here give a table showing the comparative coefficients of quality assigned to them by the engineers of the French Department of the Ponts et Chaussées.[32]