466 Farm Mechanics. 



which break into hard, clean blocks and fragments with 

 sharp edges and little material which will rub off under the 

 fingers. Any rock which crushes readily into an earth- 

 like or sandy material will not answer for road work. 



When a good road limestone wears down under the 

 wheels, the horses' feet or the roller, a loam-like powder is 

 formed which holds the right amount of water for good 

 binding, and besides this it appears more quickly to pass 

 into that cementing stage which in nature cements beds of 

 loose fragments into rock. 



The chief objection to limestone as a road metal is its 

 softness, which permits it to wear away rapidly, leaving 

 the surface dusty in dry and muddy in wet weather. 



The extremely hard and brittle quartzite which throws 

 off angular bits under the blows of horses' feet and the roll- 

 ing of wheels makes one of the poorest road materials be- 

 cause it too nearly possesses glass-like brittleness and the 

 dust is too coarse and sand-like to hold the needed water for 

 binding. 



580. Foundation and Surfacing Stone May be Different. 

 Where there is in the locality a rock which does not make a 

 good wearing surface but which binds well, like limestone, 

 this may be used to advantage for the foundation of coun- 

 try roads, thus making it necessary to import only the wear- 

 ing surface layer. 



581. Sorting Boulders Before Crushing. In localities 

 where there are many boulders available for road work 

 it will often be practicable to sort these when hauling them 

 to the crusher in such manner as to use the lighter colored 

 varieties for the foundation, reserving all of the "nigger 

 heads" for the surface layer, and in this way increase the 

 efficiency of the material. 



582. Using Limestone for Binding. Where onl} granitic 

 rock and quartzite are available for road work and these do 

 not bind well, it will often happen that the limestone of 



