=The Ideal Road Gravel.=--The ideal road gravel is a mixture of pebbles, sand and earthy material, the pieces varying from coarse to fine in such a manner that when the gravel is compacted into a road surface the spaces between the larger pebbles are filled with the finer material. The pebbles are of a variety of rock that is highly resistant to wear so that the road surface made from the gravel will have the quality of durability. The gravel possesses good cementing properties, insuring that the pieces will hold together in the road surface. The cementing property may be due to the rock powder in the deposit or to earthy material mixed with the rock particles, or to both. Table 7 shows the results of a number of tests made upon gravels and indicates that the cementing property of the gravel does not always depend upon the clay content. =Permissible Size of Pebbles.=--The larger pebbles in the gravel are less likely to crush under loads than smaller pebbles of the same sort of rock, but if the rock is of some of the tougher varieties such as trap, there is very little likelihood of even the smaller pebbles crushing. If the pebbles are of rock of medium toughness, the smaller pebbles might be crushed under the heavier loads. It is the usual practice to permit gravel to be used for the foundation course in which the pebbles are as large as will pass a 3-1/2-inch circular screen opening, and for the wearing course, as large as will pass a 2-1/2-inch circular screen opening. If larger pebbles are allowed in the wearing course, the surface is certain to become rough after a time. If the gravel is to be placed in a single course as is a very common practice, then the maximum size should not exceed that which will pass a 2-1/2-inch circular screen opening. The Wisconsin Highway Commission has constructed a very large mileage of excellent gravel roads and the sizes specified for their roads are as follows: "_Bottom Course Gravel_.--Bottom course shall consist of a mixture of gravel, sand and clay with the proportions and various sizes as follows: "All to pass a two-inch screen and to have at least sixty and not more than seventy-five per cent retained on a quarter-inch screen; at least twenty-five and not more than seventy-five per cent of the total coarse aggregate to be retained on a one-inch screen; at least sixty-five and not more than eighty-five per cent of the total fine aggregate to be retained on a two hundred-mesh sieve." "_Top Course Gravel_.--Top course shall consist of a mixture of gravel, sand and clay with the proportions of the various sizes as follows: