The macadam road consists of a mass of angular fragments of rock deposited usually in layers upon the roadbed or prepared foundation and consolidated to a smooth, hard surface produced by the passage of vehicles or by use of a road roller. The thickness of this crust varies with the soil, the nature of the stone used, and the amount of traffic which the road is expected to have. It should be so thick that the greatest load will not affect the foundation. The weight usually comes upon a very small part of the surface, but is spread over a large area of the foundation, and the thicker the crust the more uniformly will the load be distributed over the foundation. Macadam earnestly advocated the principle that all artificial road-building depended wholly for its success upon the making and maintaining of a solid dry foundation and the covering of this foundation with a durable waterproof coating or roof of broken stone. The foundation must be solid and firm; if it be otherwise the crust is useless. A road builder should always remember that without a durable foundation there is no durable road. Hundreds of miles of macadam roads are built in the United States each year on unimproved or unstable foundations and almost as many miles go to pieces for this same reason. Says Macadam: "The stone is employed to form a secure, smooth, water-tight flooring, over which vehicles may pass with safety and expedition at all seasons of the year. Its thickness should be regulated only by the quality of the material necessary to form such a flooring and not at all by any consideration as to its own independent power of bearing weight.... The erroneous idea that the evils of an underdrained, wet, clayey soil can be remedied by a large quantity of materials has caused a large part of the costly and unsuccessful expenditures in making stone roads." The evils from improper construction of stone roads are even greater than those resulting from the use of improper material. Macadam never intended that a heterogeneous conglomeration of stones and mud should be called a macadam road. The mistake is often made of depositing broken stone on an old road without first preparing a suitable foundation. The result, in most cases, is that the dirt and mud prevent the stone from packing and by the action of traffic ooze to the surface, while the stones sink deeper and deeper, leaving the road as bad as before. Another great mistake is often made of spreading large and small stones over a well-graded and well-drained foundation and leaving them thus for traffic to consolidate. The surface of a road left in this manner is often kept in constant turmoil by the larger stones, which work themselves to the surface and are knocked hither and thither by the wheels of vehicles and the feet of animals. These plans of construction cannot be too severely condemned.