334 FARM DEVELOPMENT 



for road building. This, however, is not always true, for numerous 

 examples can be shown where trap rock having the above proper- 

 ties in the highest degree has failed to give good results on light 

 traffic roads. The reason trap rock has gained so much favor with 

 road builders is because a large majority of macadam roads in 

 our country are built to stand an urban traffic, and the traps stand 

 such a traffic better than any other single class of rocks. There 

 are, however, other rocks that will stand an urban traffic perfectly 

 well, and there are traps that are not sufficiently hard and tough 

 for a suburban or highway traffic. The granites are generally 

 brittle, and many of them do not bind well, but there are a great 

 many. which when used under proper conditions make excellent 

 roads. The felsites are usually very hard and brittle, and many 

 have excellent binding power, some varieties being suitable for 

 the heaviest macadam traffic. Limestones generally bind well, are 

 soft, and frequently hygroscopic. Quartzites are almost always 

 very hard, brittle, and have very low binding power. The slates 

 are usually soft, brittle and lack binding power. 



There are but two ways in which the value of a rock as a road 

 material can be accurately determined. One way, and beyond 

 all doubt the surest, is to build sample roads of all the rocks 

 available in a locality, to measure the traffic and wear to which 

 they are subjected, and keep an accurate account of the cost both 

 of construction and annual repairs for each. By this method 

 actual results are obtained, but it has grave and obvious disad- 

 vantages. It is very costly, especially so when the results are 

 negative, and it requires so great a lapse of time before results are 

 obtained that it cannot be considered a practical method when 

 macadam roads are first being built in a locality. Further than 

 this, results thus obtained are not applicable to other roads and 

 materials. Such a method, while excellent in its results, can only 

 be adopted by communities which can afford the necessary time 

 and money, and is entirely inadequate for general use. 



The other method is to make laboratory tests of the physical 

 properties of available rocks in a locality, study the conditions 

 obtaining on the particular road that is to be built and then select 

 the material that best suits the conditions. This method has the 

 advantages of giving speedy results and of being inexpensive, and 

 as far as the results of laboratory tests have been compared with 

 the results of actual practice they have been found in the majority 

 of cases to agree. 



These tests can be made without expense to local authorities, 

 as the Office of Public Roads in the Department of Agriculture 

 maintains at Washington a complete laboratory in which is tested, 

 free of charge, all samples of road materials submitted by any 

 officer in charge of public road construction in the United States. 



Placing the layers of macadam roadway. The second 

 layer of macadam is placed on the well-rolled first 



