The material constituting the road-covering should be hard, tough, and durable, non-absorbent of water, free from clay or other earthy admixture which would interfere with the proper interlocking of the broken stone, and capable of yielding good cubical fragments with the minimum of spalls and splinters. The best road-coverings are furnished by quartzite, the basaltic, doloritic, and other trap-rocks (greenstone), and the hardest and toughest feldspathic rocks, such as porphyry, syenitic granite, etc.
Gravel Roads.—Where good material for macadamized roads is not accessible, gravel afforcis a fair sUbstitute; indeed, with proper attention to construction, this material possesses all the essential requirements of a good roadi,vay. Gravel roads, if subjected to heavy traffic, are inferior to those of broken stone, but for light traffic and where they are properly maintained, they are in all respects admirable. They are very popnlar for suburban and park roadways. The directions laid down by Gillmore in his treatise on Roads, Streets, ana' Pavements concise] y state the conditions to be observed in the building of a good road of this material. This au thority calls special attention to the capital distinction that should be made between gravel that will pack under travel and clean rounded gravel that will not pack. The packing quality of the first sort is due to the presence in it of a small proportion of clayey or earthy material; while seaside and river-sand gravel, being composed almost entirely of water-worn and rounded pebbles of all sizes, which slide easily upon one another, will not pack unless some material possessing a binding quality be mixed with it. Pit-gravel, on the other hand, usually contains too much earthy matter. The gravel for the top layer, at least, should be hard and tough; otherwise, the material will soon be pnlverized, making the road very dusty in dry weather and very muddy in wet weather. It should be composed of particles from ;< to '34 inches in dimensions, and should contain enough sandy and clayey loatu to bind the particles together firmly.
Constraction of Gravel Roads.—In preparing- the bed of a gravel road, an excavation should be made to the depth of ro or r 2 inches, and of the proper width, to receive the gravel. This is where the road-bed is in soil. Where the bed is rock, it is recommended to interpose between it and the gravel a layer of earth, to prevent the gravel from wearing too rapidly. The sur
face of the sub-grade thus excavated should preferably be given the same slope from centre toward the sides as that intended to be given to the road surface. The bottom layer should be of unscreened gravel, about 4 inches thick, evenly raked on the entire surface and then compacted by the 3se of a road-roller. The roller used for this purpose and for the preliminary con solidation of the upper layers should not be heavier than one and one-half to two tons; a much heavier roller (five to seven tons) should be used for the top layer. When the bottom layer has been made tolerably firm, a second layer of 3 or 4 inches' thickness is laid on, and treated in the manner above described; and this operation is repeated until the roadway has re ceived the required height and form. The top layer should be of screened gravel of the quality spoken of above, and the thickness of the entire bed, 3.vhen properly consolidated by rolling, should be about 12 inches. Pro vision for drainage must be made by means of side-ditches; and where such a road has been constructed upon soil of a loose or porous texture, these will usually suffice to drain the road thoroughly to the depth of a foot or more below the bottom of the bed; where the sub-soil will not admit of such free drainage, this should be facilitated by cross-clrains introduced at proper intervals below the road-covering.
Deterioration of stone roadway is no sooner given over to traffic than it is subjected to destructive influences. These are partly physical and chemical, due to the atmosphere and weather, but chiefly mechanical, due to the wheels of vehicles and the hoofs of horses, which abrade and crush to pieces the superficial layer of stone. In dry weather this pulverized material forms a disagreeable dust which the rain converts into mud. If this is allowed to accumulate in considerable amount, it seriously impedes traffic by forming ruts, and acts injuriously upon the road itself by retaining moisture upon its surface. A thin coat ing of pulverulent material is not objectionable—on the contrary, may even be advantageous as a species of protection to the surface of the road way; but when this has exceeded a certain limited thickness, its removal becomes absolntely necessary.