As the mixing progresses it may appear that patches here and there are deficient in either clay or sand and the mixture in these places is corrected by the addition of a little sand or clay as may be required. If the top-soil is used it is deposited on the sand in the required quantity and is remixed in place to insure uniformity. If either sand or clay is needed to give a satisfactory mixture, the proper material is added and mixed in as the work progresses. The surface is finally smoothed by means of the grader and drag. =Sand-Clay on Clay or Loam.=--If the existing road is of clay or loam, ample drainage will be required as discussed in Chapter IV. The surface may be constructed of a natural sand-clay mixture or of a sand mixed with the natural soil. If the former, the surface of the existing road is prepared by grading so as to insure good drainage and the natural mixture is then deposited and the surface completed as described in the preceding section. If the surface is formed by mixing sand with the existing soil, the sands may be deposited in a layer about six inches thick which will gradually mix with the soil as the road is used. A second application of sand may follow in a year or two if it is needed. Such a road surface will lack uniformity of composition and it seems preferable to mix the sand with the soil by plowing and discing as previously described. =Characteristics.=--Sand-clay road surfaces do not have sufficient durability for heavily traveled highways, but will be satisfactory for a moderate amount of traffic. These surfaces have maximum serviceability when moist, not wet, and consequently are not as durable in dry climates as in humid areas. They are likely to become sticky and unstable in continued wet weather and to become friable and wear into chuck holes in long continued dry weather. At their best, they are dustless, somewhat resilient and of low tractive resistance. GRAVEL ROAD SURFACES [Illustration: Fig. 15.--Cross Sections for Gravel Highways]