ROADS AND BRIDGES 323 



and sand are found mixed in the proportion of about 

 one part of clay to three parts of sand, or when this mix- 

 ture is artificially made, the road is very much improved. 

 (See note on cost of sand-clay roads, page 295.) 



Gravel as surfacing material. While gravel does not 

 make as durable roads as crushed stone, it is prepared 

 much cheaper, is very widely distributed, and can be so 

 cheaply procured, in many cases, that it is our most 

 widely useful road-surfacing material. 



Very many grades or forms of gravel are to be found ; 

 some coarse, others fine; some round, others subangular; 

 some soft, as limestone pebbles ; others hard, as pebbles 

 of granite or trap rock. The sharper and harder the gravel 

 the better, as a rule. The size which is most desirable 

 differs somewhat with the hardness, form and other 

 characters of the gravel. Some gravels have clay and 

 other binding materials mixed in with them. In some 

 cases gravel has been found containing sufficient iron so 

 that the roadbed composed of it became cemented and 

 hardened into a stonelike crust. 



Stones which are valuable for roads. The trap rock 

 of the palisades near New York City, owing to its hard- 

 ness and wearing ability, is freighted hundreds of miles 

 by canal and by rail to be used on road surfaces. The 

 immense deposits of trap rock at Duluth and at other 

 points in Minnesota will likewise be of great value in 

 making roads in the west. The rock at Duluth might 

 be transported by rail ; and by boat it could be cheaply 

 freighted to Milwaukee, Chicago, Detroit, Buffalo and 

 other cities on the great lakes. From Taylors Falls, in 

 addition to the railways, the St. Croix and Mississippi 

 rivers furnish a cheap waterway for floating trap rock 

 to the cities along the banks of the Mississippi river. 



The difficulty in making macadam and telford roads 

 is the immense cost of quarrying, crushing and trans- 

 porting the heavy rock. While there are many streets 



