ROADS AND BRIDGES 333 



Binding power is thus, probably, the most important property to 

 be sought for in a road-building rock, as its presence is always 

 necessary for the best results. The hardness and toughness of 

 the binder surface more than of the rock itself represents the hard- 

 ness and toughness of the road, for if the weight of traffic is suffi- 

 cient to destroy the bond of cementation of the surface, the stones 

 below are soon loosened and forced out of place. When there is 

 an absence of binding material, which often occurs when the rock 

 is too hard for the traffic to which it is subjected, the road soon 

 loosens or ravels. 



Experience shows that a rock possessing all three of the proper- 

 ties mentioned in a high degree does not under all conditions make 

 a good road material; on the contrary, under certain conditions 

 it may be altogether unsuitable. As an illustration of this, if 

 a country road or city parkway, where only a light traffic prevails, 

 were built of a very hard and tough rock with a high cementing 

 value, the cheapest results would not be obtained. Such a rock 

 would so effectively resist the wear of a light traffic that the amount 

 of fine dust worn off would be carried away by wind and rain faster 

 than it would be supplied by wear. Consequently, the binder 

 supplied 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 very expensive. 



There are some rocks, such as limestones, that are hygroscopic, 

 or possess the power of absorbing moisture from the air, and in 

 dry climates such rocks are distinctly valuable, as the cementation 

 of rock dust is in a large measure dependent for its full develop- 

 ment on the presence of water. The degree to which a rock 

 absorbs water may also be important, for in cold climates this to 

 some extent determines the liability of a rock to fracture by freez- 

 ing. It is not so important, however, as the absorptive power 

 of the road itself, for if a road holds much water the destruction 

 wrought by frost is very great. This trouble is generally due to 

 faulty construction rather than to the material. The density or 

 weight of a rock is also considered of importance, as the heavier 

 the rock the better it stays in place and the better it resists the 

 action of wind and rain. 



Rocks belonging to the same species and having the same name, 

 such as traps, granites, quartzites, etc., vary almost as much in 

 different localities in their physical road-building properties as they 

 do from rocks of distinct species. This variation is also true of 

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

 in the size 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 rock possess in common some road-building properties. For 

 instance, the trap rocks as a class are hard and tough and usually 

 have binding power, and consequently stand heavy traffic well; 

 and for this reason they are frequently spoken of as the best rocks 



