44 THE TESTING OF ROAD MATERIALS. 



the first case a softer rock should be used; in the latter, a harder, 

 tougher rock, and in all cases a rock of high binding power. For 

 example, it' a country road or city park way, where only a light traffic 

 prevails, were built of a very hard and tough rock with a high cement- 

 ing value, neither the best results nor the cheapest, if a softer rock 

 were available, would be obtained. Such a rock would so effec- 

 tively resist the wear of a light traffic that the amount of line dust 

 worn off would be carried away by wind and rain faster than it would 

 be supplied by wear. Consequently the amount of binder produced 

 by wear would be insufficient, and if, not supplied from some other 

 source the road would soon go to pieces. The first cost of such a rock 

 would in most instances be greater than that of a softer one, and the 

 necessary repairs resulting from its use would also be expensive. 



It has already been pointed out in a former publication" from this 

 laboratory that rocks belonging to the same species and having the 

 same name, but coming from different localities, vary from each other 

 in their physical road-building properties almost as much as they do 

 from rocks of distinct species. Wide variation also occurs in the 

 mineral composition of rocks of the same species, as well as in the 

 si/e and arrangement of their crystals. It is impossible, therefore, 

 to classify rocks for road building by simply giving their specific 

 names. It can be said, however, that certain species of rocks pos- 

 8688 in common some road-building properties. For instance, the 

 trap 6 rocks as a class stand heavy traffic well, as they are hard, 

 tough, usually have binding power, and are therefore frequently 

 spoken of as the best rocks for road building. This, however, is 

 not always true, for numerous examples can be shown in which trap 

 rocks having the above properties in the highest degree have 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 trallic. and the traps >tand such a traffic better than any other 

 single class of rocks. There are. however, other rocks that will stand 

 an urban trallic perfectly well, and there are traps that are not suffi- 

 ciently hard and tough for a suburban or highway trallic. The granites 

 are gem-rally brittle and nuiny of them do not hind well but there are 

 a great many which when used under proper conditions make excel- 

 lent road-. The feUite- are usually very hard and brittle and many 



["he Selection of Materials for Macadam Ilmnls. Vcarl.<..,k of the Department of 

 .Virriculture, 1^00. 



''This term is derived from the S\vedi>h \\ord tru/ijm, meaning steps, and was 

 originally applied to the crystalli/.cd basalts of the coast of Sweden, which much 

 resemble steps in appearance. As now used l.y road builders, it embraces a large 

 variety of igneous rocks, chiefly those of line crystalline structure and of dark-blue, 

 \rray. and ^reeii colors. They are generally diabases, diorites, trachytes, and basalts. 





