8 BULLETIN 370^ U. S. DEPAETMENT OP AGEICULTUfiE. 



CHERT. 



Chert is a very hard material, but frequently shows a low resist- 

 ance to wear, owing to its tendency to fracture along lines which 

 have developed as shrinkage cracks in the rock structure. For this 

 reason it is extremely difficult to test for toughness. The cementing 

 value of pure chert is usually low, but some highly weathered deposits 

 develop in service good cementing value, especially if a high-binding 

 clay is associated with it. Comparatively few samples which have 

 been submitted for examination have been found suitable for all tests. 

 Of those examined, however, the French coefficient of wear has 

 usuall}' been found to lie between 2 and 8, with an average of 5; 

 toughness between 7 and 26, with an average of IG; and the hardness 

 coefficient between 19 and 20. Specific gravity usually lies between 

 2.4 and 2.65 and the average weight per cubic foot is about 160 

 pounds. Water absorption ma}^ run from a few tenths of 1 per cent 

 to over 8 per cent. 



SHALE AND SLATE. 



Shales and slates are highly laminated rocks that tend to break 

 into fiat plates not suitable for road-building purposes. They are 

 seldom used in road construction, except perhaps as a filling for sub- 

 foundations. They vary gi-eatly in nearly all of their physical 

 properties. 



RARE ROAD-BUILDING ROCKS. 



A comparatively few samples of a number of families of rocks 

 which are occasionally used in road building have been examined in 

 the laboratories of the United States Office of Public Eoads and 

 Rural Engineering. They need not be considered in detail, but the 

 usual ranges as well as the averages of results of the more important 

 physical tests of these rocks are given in Table I. 



Table I. — Tlie rare road-bidlding rocks. 



Num- 

 ber of 



Name. 



French coefficient 

 of wear. 



Toughness. 



Hardness. 



sam- 

 ples. 



Ordinary 

 range. 



Aver- 

 age. 



Ordinary 

 rango. 



Aver- 

 age. 



Ordinary 

 range. 



Aver- 

 age. 



20 



Amphibolite 



11.3-26.7 

 12. 7-22. 7 

 10.0-18.7 

 11.9-21.3 

 7.6-13.2 

 2.6-14.2 

 11.5-23.5 

 7.0-18.7 



16.7 

 16.1 

 13.0 

 15.8 

 10.3 

 10.1 

 16.2 

 13.1 



12-40 

 14-28 

 10-23 



19 

 26 

 16 

 16 

 10 

 14 

 22 



16.6-19.0 

 18.4-19.3 

 17.6-19.5 



17.5 



10 



Eologite 



18.5 



12 





18.0 



11 



Felsite 



18.7 



6 





9-12 

 11-21 

 21-34 



8-22 



13.3-16.6 

 18. 3-18. 6 

 17. 7-19. 1 

 17.3-19.2 



15.0 



8 





18.4 



5 





18.1 



19 



Syenite 



IS.l 









