ROADS AND BRIDGES 339 



screenings most of it will sift to the bottom of the first course as 

 soon as the screenings are spread, and its value as a binding 

 material will be partially lost. 



Cost of crushed rock. In large quantities trap rock 

 could be placed on cars or boats at such shipping points 

 as Taylors Falls or Duluth, Minn., at a very low price 

 per cubic yard, as could also granite at St. Cloud, Minn. 

 Large contracts have been filled at the Palisades, New 

 York, at a very low price per ton. Trap rock can be 

 carried by boat from 

 Duluth to Chicago or Buf- 

 falo, sometimes as ballast, 

 at a small price per ton. 

 For each mile of macadam 

 road, 12 feet wide and 10 

 inches deep, about 2,000 



riiKir vurrlc nf rnrk are Figure 226. Reversible road rollers to be 



CUDIC yaras OCK are drawn by horses built to weigh 2 to 6 tons 



needed. If the lower 7 



inches are of limestone and the upper 3 inches of trap 

 rock, 1,400 cubic yards of limestone and 600 cubic 

 yards of trap rock are required. At common prices for 

 freight these amounts make the use of stone roads 

 impossible except in occasional much-used roads where 

 the people have the means for the large expenditure 

 required. 



Quantities of crushed rock required for different widths and 

 depths. The following table, which is quoted from the Report 

 of the State Commissioner of Public Roads of New Jersey, approx- 

 imates the number of tons of rolled stone required to construct 

 a mile of road of the various widths and depths. The New Jersey 

 Commissioner says in explanation of the table: 



"The basis is 3,000 tons of loose stone or 3,500 tons of com- 

 pressed stone for a road one mile long, 16 feet wide and 8 inches 

 deep. A road 8 inches deep, when finished, will have required at 

 least 10 inches of stone. It should be placed in two layers of 5 

 inches each, and each layer rolled down to 4 inches. Then the 

 application of the f inch and screenings will bring the road to the 

 prescribed depth; for other thicknesses the stone should be placed 

 in proportion to the intended finished depths." 



