68 JOURNAL OF THE 



These ''co-efficients," showing tlie relative quality of 

 various road materials, were obtained by French engineers 

 as the result of an extended series of tests, and were found 

 to agree fairly well with the results arrived at by actual 

 observation of the wear of materials in the roads. The 

 co-efficient 20 is equivalent to "excellent," 10 to "suffi- 

 ciently good," and 5 to "bad." 



Stones not Suitable as Road Material. — Before pro- 

 ceeding to the consideration of the stones found in North 

 Carolina adapted to use as road material it may be well to 

 consider briefly some of those that are not suited to this 

 purpose. In general, it may be said that all schistose and 

 slaty rocks, /. e., all rocks which split or break easily into 

 layers or flakes, should be discarded. No rock of what- 

 ever species which is already in the advanced stages of 

 decay, so as to become crumbly and soft or porous, should 

 be used in macadamizing roads, as the result in all such 

 cases will be that, under the action of the wheels and 

 hoofs, these materials become ground into fine powder, 

 which becomes mud when wet, and dust when dr\'. There 

 are many places, however, where a decayed granite or 

 gneiss rock, when highly siliceous, will make a good foun- 

 dation for a Macadam road, and will be found useful as a 

 covering on clay in the improvement of dirt roads. There 

 are other materials, like quartz (" white flint"), which are 

 hard enough, but which are quite brittle, and hence easily 

 crushed to powder, and which, consequently, should not 

 be used when better material is available. Sandstones, as 

 a rule, are unfit for use in macadamizing roads, as they are 

 easily crushed and usually porous. 



Stones Suitable as Road Material. — "The materials 

 used for broken-stone pavements 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 onl\- moderate. If, howe\'er, the traffic is 



