Page:Historic highways of America (Volume 15).djvu/177

 lacks sufficient hardness or toughness to meet the traffic to which it is subjected. 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