464 Farm Mechanics. 



with a road grader or road machine. With this tool the 

 surface of the road-bed is prepared at the same time and the 

 shoulders left in such shape that very little hand labor will 

 be required for the finishing touches. After the. shoulders 

 have been roughly formed and before the finishing touches 

 are given the roller should go over the road-bed to make 

 sure that it is properly firmed and that there are no soft 

 places. 



579. Kinds of Rock for the Road. Practical experience 

 has demonstrated that the best rocks for road making are 

 the dark green, black and dark gray trap or igneous rock 

 such as are known in common language as "nigger heads" 

 in glaciated countries where large boulders are common in 

 the fields and cuts of roads. They are tough, fine grained 

 rock, much less brittle than most others, which yield when 

 grinding upon themselves and under the wheel a fine rock 

 flour whose texture is such that it holds the needed amount 

 of moisture to make it bind together well, and consequently 

 a road built from these fragments sets sooner than almost 

 any other crystalline rock and hence is subject to less in- 

 ternal wear. 



Next to the trap rock in value for road building purposes 

 stand the closer grained hornblencl-bearing syenites and 

 gneisses which are species of granite where hornblend takes 

 the place of mica of the true granites. It is the class of 

 dark minerals allied to hornblend composing much of the 

 trap rock referred to above which makes that the best road 

 stone. 



Next in order stand the true granites made up of quartz, 

 feldspar and mica, and their gncissoid varieties. The best 

 of this class of rocks are the close fine-grained varieties 

 having the least tendency to break into thin layers, giving 

 flat instead of cubical blocks. 



To the granites and syenites with their banded or gneiss- 

 oid varieties belong the lighter colored and flesh colored 

 boulders which are usually associated with the "nigger 

 heads" of glacial drift. 



