ROADS 477 



and kept in proper repair. If this is not done, the money spent 

 is more than wasted. It is more economical, as a rule, to bring 

 good materials a long distance by rail or water than to employ 

 inferior ones procured close at hand. 



The durability of roads depends largely upon the power of the 

 materials of which they are composed to resist those natural and 

 artificial forces which are constantly acting to destroy them. 

 The fragments of which they are constructed are liable to be 

 attacked in cold climate by frost, and in all climates by water 

 and wind. If composed of stone or gravel, the particles are 

 constantly grinding against each other and being exposed to the 

 impact of the tires of vehicles and the feet of animals. Atmos- 

 pheric agencies are also at work decomposing and disintegrating 

 the material. It is obviously necessary, therefore, that great 

 care be exercised in selecting for the surfacing of roads those 

 stones which are less liable to be destroyed or decomposed by 

 these physical, dynamical and chemical forces. 



Useful stones for road-huilding. — Siliceous materials, those 

 composed of flint or quartz, although hard, are brittle and defi- 

 cient in toughness. Granite is not desirable because it is 

 composed of three materials of different natures, viz., quartz, 

 feldspar, and mica, the first of which is brittle, the second liable 

 to decompose rapidly, and the third laminable or of a scaly or 

 layerlike nature. Some granites which contain hornblende 

 instead of feldspar are desirable. The darker the variety the 

 better. Gneiss, which is composed of quartz, feldspar, and mica, 

 more or less distinctly slatey, is inferior to granite. Mica-slate 

 stones are altogether useless. The agrillaceous slates or clayey 

 slates make a smooth surface, but one which is easily destroyed 

 when wet. The sandstones are utterly useless for road building. 

 The tougher limestones are very good, but the softer ones, 

 though they bind and make a smooth surface very quickly, are 

 too weak for heavy loads ; they wear, wash, and blow away very 

 rapidly. 



The materials employed for surfacing roads should be both 

 hard and tough, and should possess by all means cementing and 

 vecementing qualities. For the Southern States, where there are 

 no frosts to contend with, the best qualities of limestone are 

 considered quite satisfactory so far as the cementing and rece- 

 menting qualities are concerned, but in most cases roads built of 

 this class of material do not stand the wear and tear of traffic 

 like those built of trap rock, and when exposed to the severe 

 Northern winters such material disintegrates very rapidly. In 



