ECONOMIC GEOLOGY 205 



the same composition as augite which occurs in trap; and a horn- 

 blende granite should be a very good road metal. Where horn- 

 blende 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 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 includes 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 a carbonate of lime, containing also magnesia, alumina 

 and silica. It has been suggested that it might be used profitably 

 as a binder with stone of less binding power. 



Sandstone has usually no lime, magnesia or alumina and there- 

 fore has no binding properties and never makes a first rate road, 

 as the fragments continually break loose. 



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. 



In New York the best road materials occur in certain limited 

 areas, and at points distant from these the cost of transportation 

 is the controlling feature. 



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

 and used in all places where their cost is not prohibitory. Ex- 

 perience shows, however, that unless these materials are used 

 under the direction of experienced road engineers, they are less 

 satisfactory than limestone, and when it is proposed to mac- 

 adamize 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 subgrade 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 



