ROAD MATERIALS AND THEIR DISTRIBUTION 105 



very good road metal. Where hornblende is absent one would expect to 

 find less binding power. 



Granite is harder than trap and therefore should resist wear better, but 

 this quality is offset by its usually smaller binding power due to the 

 presence of quartz so that trap should be preferred as a rule. 



Granite is found in the Adirondack region and in the Highlands of the 

 Hudson, also in Westchester county. The commercial term granite in- 

 cludes various kinds of gneiss. 



Magnesian limestone has great binding power but is quite soft and 

 therefore not very durable for heavy traffic. Chemically, this rock is 

 chiefly a carbonate of lime also containing carbonate of magnesia, alumina 

 and silica. Limestone entirely free from magnesia is rare. 



It has been suggested that this stone may be used profitably as a binder 

 over stone of less binding power. 



Limestone is found chiefly in areas parallel to and near the main line 

 of the New York Central railroad and in a zone around the Adirondacks. 



Sandstone consists chiefly of quartz, has usually no lime, magnesia or 

 alumina and therefore has no binding properties and never makes a first 

 rate road, as the fragments continually break loose. 



In New York the best road materials occur in certain limited areas, 

 and at points distant from these the cost of transportation is the con- 

 trolling feature in the question of their use. 



The accompanying map shows the distribution of the areas of rock 

 already mentioned which are available for road construction in New 

 York. 



For high class road building, trap and granite will be preferred and 

 used in all places where their cost is not prohibitory. Experience shows, 

 however, that unless these materials are used under the direction of ex- 

 perienced road engineers, they are less satisfactory than limestone, and 

 when it is proposed to macadamize a road by simply covering it with 

 broken stone, the latter though less durable, will be more satisfactory. 



When granite and trap are properly laid, on a well prepared bed and 

 rolled with a heavy steam roller to the proper standard of firmness, 

 nothing can be better, but where no steam roller is available and the sub- 

 grade is not properly prepared, the trap and granite are liable to afford 

 only an unpleasant and uneven surface of hard angular fragments which 

 ceaselessly roll about on the surface of the road injuring the horses and 

 making pleasure driving impossible. 



Limestone from its softness and greater binding power is more easily 

 rolled into an even surface under the wheels of vehicles, and while not 



