Stone Roads. 465 



The chief difficulty with syenites and gTanites for road 

 metal is their brittle, unyielding 1 (juality and coarse crystal- 

 line structure which makes them grind and pound up into 

 a coarse sand without a sufficient amount of the finest dust 

 to give it the needed Water-holding ]><>\ver to permit it to 

 properly bind the pieces together. The road-bed fails to 



FIG. 220. View showing where four inches of crushed rock for wearing surface 

 is being built upon four inches of road-gravel as foundation layer. 



set quickly and the internal wear is larger while there is a 

 greater tendency for ruts to form in wet weather and for 

 the surface to ravel or throw out loose pieces in a dry time. 

 Next to the syenites and granites in irriirral availability 

 for road metal stand the close grained hard limestones 



