482 EEPOET OF NATIONAL MUSEUM, 1899. 



7. ROAD-MAKING MATERIALS. 



Roadways subject to any considerable amount of traffic demand 

 almost invariably some sort of stone bedding to prevent their becom- 

 ing soft or badly cut up and rutted by wheels and hoofs of horses. 

 Until within a comparatively few years it has been the general custom 

 to pave the streets of cities and towns with rectangular blocks of 

 granite, trap, or other hard rock, forming thus the well-known Belgian 

 block and Telford pavements. Such are set in regular rows and the 

 interspaces filled with sand and sometimes with tar or asphalt. For 

 suburban and country roads a pavement of broken stone, the invention 

 of a Mr. L. Macadam about 1820, and known by his name, is at pres- 

 ent the most extensivel} T used. The invention is based upon the prop- 

 erty possessed by freshly broken stone of becoming compacted and to 

 a certain degree even cemented when subject to heavy rolling and the 

 impact of wheels. The finer particles, broken away by the action of 

 the wheels, fill the interstices of the larger, and gradually bring about 

 an induration forming a roadbed hard, smooth, and durable. 



Not all materials are equa% good for macadamizing purposes. If 

 the rock is too hard ordinary travel is not sufficient to produce the 

 desired amount of fine material, and satisfactory cementation does 

 not ensue. If too soft it grinds away too rapidly. If the material is . 

 decomposed, it is stated, it does not become sufficiently indurated 

 refuses to set, as it were. 



Obviously the bulk matter of any roadbed must be built up of 

 materials from near-by sources, owing to cost of transportation. For 

 surfacing, however, materials are often carried long distances. For 

 this purpose a hard, dense rock, such as the finer grades of trappean 

 rocks, are now most generally used. 



Macadam is laid with or without a foundation of larger stones. 

 When such is used a thickness of from 6 to 12 inches is recommended 

 and over this is laid from 4 to 6 inches of the broken stone or "metal." 



Taking all points into consideration, it is probable that the best size for macadam, 

 for hard and tough stones, such as basalt, close-grained granite, syenite, gneiss, and 

 the hardest of primary crystallized rocks, is from 1 to 1J inches cube, according to 

 their respective toughness and hardness, while stones of medium quality ought to 

 be broken to gauge of from 1 to 2\- inches, and the softer kinds of stone might vary 

 between the limits of 2 and 1\ or 2J inches, but the latter is a size which should 

 seldom be specified. 



On roads for light driving it is customary to place a final surfacing 

 of smaller stone, such as will pass a 1-inch mesh. 



Considerable importance is attached to the manner in which the macadam is pre- 

 pared for use. Machine-broken stone is not considered of the same value as that 

 broken by hand. The stones are not so regular a size and shape, and there is a 

 greater proportion of inferior stuff. A mechanical crusher is apt to stun the mate- 

 rial, and does not leave the edges so sharp for binding as they are when the stone is 

 broken with a small hammer. 1 



1 Circular No. 12, TJ. S. Department of Agriculture, Office of Road Inquiry, 1896. 



