If, on the contrary, the fine binding material of the surface is carried off by wind and rain and is not replaced by the wear of the coarser fragments, the surface stones will soon loosen and allow water to make its way freely to the foundation and bring about the destruction of the road. Such conditions are brought about by an excess of hardness or toughness of the rock for the traffic. Under all conditions a rock of high cementing value is desirable; for, other things being equal, such a rock better resists the wear of traffic and the action of wind and rain. This subject, however, will be referred to again. Until comparatively recent years but little was known of the relative values of the different varieties of rock as road material, and good results were obtained more by chance and general observation than through any special knowledge of the subject. These conditions, however, do not obtain at present, for the subject has received a great deal of careful study, and a fairly accurate estimate can be made of the fitness of a rock for any conditions of climate and traffic. In road-building the attempt should be made to get a perfectly smooth surface, not too hard, too slippery, or too noisy, and as free as possible from mud and dust, and these results are to be attained and maintained as cheaply as possible. Such results, however, can only be had by selecting the material and methods of construction best suited to the conditions. In selecting a road material it is well to consider the agencies of destruction to roads that have to be met. Among the most important are the wearing action of wheels and horses' feet, frost, rain, and wind. To find materials that can best withstand these agencies under all conditions is the great problem that confronts the road-builder. Before going further, it will be well to consider some of the physical properties of rock which are important in road-building, for the value of a road material is dependent in a large measure on the degree to which it possesses these properties. There are many such properties that affect road-building, but only three need be mentioned here. They are hardness, toughness, and cementing or binding power.