HIGHWAY CONSTRUCTION 67 



this foundation, contending that the subsoil, however bad, would 

 carry any weight if made dry by drainage and kept dry by an im- 

 pervious covering. The names of both have ever since been 

 associated with the class of road which each favored, as well as with 

 roads on which all their precepts have been disregarded. 



Quality of Stones. The materials used for broken-stone pave- 

 ments must of necessity vary very much according to the locality. 

 Owing to the cost of haulage, local stone must generally be used, 

 especially if the traffic be only moderate. If, however, the traffic is 

 heavy, it will sometimes be found better and more economical to 

 obtain a superior material, even at a higher cost, than the local 

 stone; and in cases where the traffic is very great, the best material 

 that can be obtained is the most economical. 



The qualities required in a good road stone are hardness and 

 and toughness and ability to resist the disintegrating action of the 

 weather. These qualities are seldom found together in the same 

 stone. Igneous and siliceous rocks, although frequently hard and 

 tough, do not consolidate so "well nor so quick as limestone, owing 

 to the sandy detritus formed by the two first having no cohesion, 

 whilst the limestone has a detritus which acts like mortar in binding 

 the stones together. 



A stone of good binding nature will frequently wear much 

 better than one without, although it is not so hard. A limestone 

 road well made and of good cross-section will be more impervious 

 than any other, owing to this cause, and will not disintegrate so 

 soon in dry weather, owing partly to this and partly to the well- 

 known quality which all limestone has of absorbing moisture from 

 the atmosphere Mere hardness without toughness is not of much 

 use, as a stone may be very hard but so brittle as to be crushed to 

 powder under a heavy load, while a stone not so hard but having a 

 greater degree of toughness will be uninjured. 



By a stone of good binding quality is meant one that, when 

 moistened by water and subjected to the pressure of loaded wheels 

 or rollers, will bind or cement together. This quality is possessed to 

 a greater or less extent by nearly all rocks when in a state of dis- 

 integration. The binding is caused by the action of water upon the 

 chemical constituents of the stone contained in the detritus produced 



