ROADS 



161 



resist wear, which requires somewhat different character- 

 istics from the ability to withstand pressure. Road stone 

 should also be tough, in order that it will not be shattered 

 by the blows to which it will be subjected. It must also, in 

 the usual macadam construction, furnish a dust which has 

 a cementing or binding power. As the stone wears, a dust 

 forms, which becomes lodged between the stone particles. 

 This dust when wet forms a sort of cement which, upon 

 hardening, holds the fragments of stone together, resembling 

 in many respects cement or concrete. 



Testing Stone for Road Construction. Nearly every 

 state maintains a highway commission which is equipped with 

 apparatus for testing road stone for the various qualities 

 mentioned. These tests are capable of determining fairly 

 and accurately just what may be expected as to durability 

 of a certain kind of stone when used in road construction. 

 The construction of stone roads is so expensive that in no 

 case should materials of doubtful value be used. 



Fig. 84. Rolling 1 the first course of stone. 



