ELISHA MITCHELL SCIENTIFIC SOCIETY. 67 



CHARACTER OF ROAD MATERIALS. 



''In considering the relative fitness of the various mate- 

 rials," savs Byrne, "^ ''the following physical and chemical 

 qualities must be sought for: 



" ( I ). Hardness, or that disposition of a solid which ren- 

 ders it difficult to displace its parts among themselves. 



"(2 '. Toughness, or that quality which will endure light 

 but rapid blows without breaking. 



"(3). Ability to withstand the destructive action of the 

 weather, and probablv some organic acids produced by the 

 decomposition of excretal matters, always present upon the 

 roadways in use. 



"(4. The porosity, or water-absorbing capacity, is of 

 considerable importance. There is, perhaps, no more 

 potent disintegrator in nature than frost, and it may be 

 accepted as fact that of two rocks which are to be exposed 

 to frost, the one most absorbent of water will be the least 

 durable." 



The following table shows absorptive power of a few 

 common stones: t 



Perceutage of Water Absorbed. Percentage of Water Absorbed. 



Granites ..- - -- 0.06100.155 Limestones--. 0.20 to 5.00 



Marbles -. o.oStoo. 16 Sandstones - .- 0.41 to 5.48 



Something of the quality and suitability of diflferent 

 materials for use in broken-stone pavements is shown in 

 the following table: 4! 



Materials. 



Co-efficients of Co-efficients of 

 Wear. Crushing. 



Basalt --- - -- 12.5 to 24.2 12.1 to 16. 



Porphyr}' -- -- 14.11022.9 8.3 to 16.3 



Gneiss -- --- 10.3 to 19.0 13.4 to 14.8 



Granite 7.3 to 18.0 7.7 to 15.8 



Svenite -- -- 11. 6 to 12.7 12.4 to 13.0 



Slag --- -- --- 14.5 to 15.3 7.2 to 1 1. 1 



Ouartzite -_- -- 13.8 to 30.0 12.3 to 21.6 



Qnartzose sandstone 14.3 to 26.2 9.9 to 16.6 



Quartz ---, 12.91017.8 12.3 to 13.2 



Limestone -_ - --; :6. 6 to 15.7 6. 5 to 13.5 



^Highway Construction, p. 24. ^Ibid.. p. 26. Xlbid., p. 172. 



