Paducah.This gravel came from a pit about two miles west of Paducah, Ky., on the Ohio river at the mouth of the Tennessee river. It makes excellent roads that pack quickly under traffic and are not much affected by freezing and thawing. The coarse material consists of water-worn chert pebbles, and is cemented by ferruginous clay. The chert is brittle and crushes with a sharp splintery fracture, and consolidates readily under traffic, the sharp angular fragments giving an immobile mass and offering excellent surfaces for the cementing action of the binder.
Rosetta. This gravel comes from the Rosetta pit at Fort Gibson, near Vicksburg, Miss., and is much used by the Illinois Central Railroad as ballast. It is here included under the belief that it will also make good wagon roads. The quartz pebbles are quite rough and angular, and in the pit seem to be quite firmly cemented together by ferruginous clay.
Conclusion.From the preceding, the following conclusions may be drawn. 1. The relation between the proportion of voids and the per cent of clay is no indication of the road-building qual ities of a gravel, for under traffic some of the fragments may crush and decrease the per cent of voids and at the same time increase the amount of the binding material. 2. The friability of the pebbles has a greater effect upon the road-building qualities of a gravel than the per cent of the voids. 3. The binding material
may be clay, or clay and iron, or pulverized limestone, or all of these combined. The less clay the more slowly will the road bind, but the less it will be affected by frost.
A study similar to the preceding will not certainly determine the suitability of a gravel for road purposes, hut it will throw valu able light upon its probable behavior in the road. The only sure way to determine the road-building qualities of a gravel is to test it by actual service, for much depends upon the friability of the pebbles, the weight of the traffic, the climatic conditions, etc. In applying the test of actual service, particularly to determine the relative merits of two gravels, account should be taken of (1) the nature of the soil, (2) the care employed in preparing the founda tion, (3) the quantity of material used, (4) the amount of traffic, (5) the care given to maintaining the road, and (6) the length of time the material has been in service. The character of a gravel mad is generally indicated by the sound of the metal tires of the wheels of the vehicles passing over it. If the wheel makes a continuous crisp gritty sound, the road is reasonably good; if the gritty sound is absent, there is probably too much earthy matter on the sur face; and if the sound is intermittent and discontinuous, there are probably too many large pebbles in the surface material.