It should not be forgotten that the limestones in Tables 19 and 20 are selected samples and are not representative of the whole series. Some limestones are entirely too soft for road material.
The limestones are the most widely diffused of any bedded rock employed for road purposes. In a large part of the Missis sippi Valley they constitute the only material, except glacial drift, with to build broken-stone roads.
Quartzite was originally sand, and has been changed into a com pact mass by pressure and heat. The quartzites differ greatly, but are generally too soft or have too little cementing power to make a good road-building material. They are confined to com paratively "limited areas, being found principally- in the moun tainous districts of the Appalachian and Cordilleran areas and in the Adirondacks and the Ozarks." * Quartz when found in large veins sometimes makes a fair road metal. It is very hard and breaks with sharp edges, but readily crushes into fine dust which has no binding power. Roads of this material rarely attain a smooth surface and always wear very rapidly.
Chert, a variety of quartz, is a siliceous material with the characteristics of flint, but differing from it in being of a tougher nature, and in breaking with a splintery instead of a conchoidal fracture. Chert has a variety of colors—red, yellow, gray, and brown. It is usually mixed with more or less lime, and beds of chert often grade imperceptibly into limestone. Chert varies greatly in quality, and being brittle and somewhat deficient in bind ing power it is never a first-class road stone, but will usually give fairly good results. It is often of great value, since it occurs in those parts of the country where good road-building material is scarce or entirely absent. Chert is not usually found in a solid mass, but in a finely divided state, broken ready for placing upon the road. Chert has already been considered as a gravel (see § 239).
" Where the glacial bowlders have lain since the ice time exposed to the weather, or where, being of small size, they lie in the zone to which decay has penetrated from the surface, they are often so far decomposed as to be essentially unfit for use in road-making, save it may be as telford pavement, or as a bottom coating of broken stone which is to be covered with material of a better grade.
The variation in the extent to which decay has injuriously affected the surface stone may be judged by simple tests which may be readily applied. After a brief experience a judicious person with a light sledge hammer can, by striking the stones, readily deter mine the state of the masses. If they ring sharply to the blow they may be judged sufficiently sound. If, however, they pul verize under the successive strokes, or if they show evident traces of decay, as by iron stains penetrating the mass, they may be con demned as a source of supply." * There is inevitably a good deal of difference in the road material obtained from bowlders and pebbles, and care should be taken not to mix the hard and soft varieties, as otherwise the road will wear unevenly. If the different varieties are laid in distinct sec tions, valuable knowledge may thus be obtained as to the wearing qualities of the different grades.
Shale is the most abundant of all stratified rocks. When the clay is nearly pure it is called argillaceous or clay shale; and when it contains considerable sand, it is known as arenaceous or sandy shale. The shales vary in color, being gray, reddish, or very black. For road-building purposes the argillaceous shales are en tirely worthless, and the sandy shales are useful only for a top dressing,—and are not very good for that.
Slate is often quite hard to the quarryman's tools, but softens rapidly in contact with water. As a road material the fragments quickly grind to dust which has but slight binding power. Slate makes a smooth road, but one that wears very rapidly, particularly when wet. It is sometimes used as a surfacing or binding ma terial, but is much inferior to clean sand or good stone dust.