Stone Roads. 467 



the locality may be crushed fine to form screenings and 

 used to great advantage as a binding material to hold the 

 harder rocks more securely in place. This practice would 

 be especially desirable for the foundation layer where it 

 could not be converted into dust But in localities where 

 both limestone and the harder rock are available, but where 

 the limestone can be obtained at much the less cost, this 

 may be used alone for the foundation and as a binding ma- 

 terial for the surface layer. 



583. Roads Made Without Binding Material. It was Ma- 

 cadam's practice in road building to strictly forbid the use 

 of all binding material whatsoever. He preferred to wait 

 for the general traffic over the road to develop from the 

 wear of the crushed stone, both superficial and internal, 

 the necessary amount of rock flour to do the work of filling 

 and cementing. While this work was in progress the road 

 was given constant supervision to keep it in proper form. 

 At the same time the filling and binding material was be- 

 ing slowly produced there was brought upon the road with 

 the wheels and horses' feet a considerable amount of earth 

 which slowly worked downward and united with the rock 

 flour to complete the consolidation. Macadam certainly 

 secured in the end a better road by this method than was 

 usually secured with the use of the then available binding 

 material. 



It must be remembered, however, that in his time rock 

 were crushed by hand and little fine material was made to 

 use for binding, whereas with the modern rock crushers a 

 large amount of this material is produced which must be a 

 dead loss if it cannot be used for binding and surfacing, 

 and it is quite certain that had Macadam used our modern 

 rock crushers he would have availed himself of the screen- 

 ings. 



584. Use of Sand for Binding. The great readiness with 

 which clean dry sand works into and fills the voids between 

 the stone of a road, the ease with which it may be handled 



