6 BULLETIN 370^ U. S. DEPARTMENT OP AGEICULTUEE. 



GRANITES. 



Granite, the typical rather coarse-grained igneous rock, is charac- 

 terized by low toughness and high hardness. The average value for 

 the former, as will be seen from the chart, is about 8, while that for 

 the latter runs as high as for the trap group, about 18.5. The abra- 

 sion test develops an average French coefficient of wear of about 11, 

 somewhat lower than for the trap-rock group. Cementing values 

 made on granites run low, as has been demonstrated by experience, 

 the only exceptions being very highly weathered material which 

 usually shows low toughness and resistance to wear. The specific 

 gravity of the granites averages close to 2.7 and is seldom less than 

 2.6 or more than 2.8. The weight per cubic foot, therefore, averages 

 168 pounds, and may ordinarily vary from 163 to 175 pounds. Water 

 absorption has been found to run from about 0.04 to 3 per cent. 



LIMESTONES AND DOLOMITES. 



The limestones and dolomites, or magnesium limestones, are un- 

 doubtedly the most widely used road-building rock. It will be seen 

 from the chart that thej^ run much, lower in hardness, toughness, and 

 resistance to wear than do the traps or granites. The average French 

 coefficient of wear is about 8, toughness 7, and hardness 15. The 

 cementing values are usually good, about 75 per cent of all samples 

 tested running over 25. The specific gravity of the limestones and 

 dolomites averages close to 2.7, about that of the granites, and is sel- 

 dom less than 2.6 or more than 2.85. In general, the weight per cubic 

 food will run from 160 to 178 pounds, with an average of about 168 

 pounds for the limestones and 170 pounds for the dolomite. Absorp- 

 tion may vary from a few hundredths of 1 per cent to over 13 per cent. 



SANDSTONES. 



The sandstones are characterized by wide variations in the results 

 of all tests. In fact, the highest and lowest values obtained for all sam- 

 ples tested have, with one exception, been upon sandstone. The aver- 

 age French coefficient of wear is about 12, average toughness about 10, 

 and average hardness about 16. The cementing value of sandstones 

 varies widely, depending upon their composition. Thus certain 

 varieties of feldspathic sandstone somewhat resembling trap rock in 

 appearance almost invariably show high binding value in the labora- 

 tory. Their sj)ecific gravity also varies between wide limits, but 

 usually lies between 2.4 and 2.8, with an average of 2.62. The weight 

 per cubic foot therefore varies from 150 to 175 pounds and averages 

 164 pounds. Absorption runs from a few hundredths of 1 per cent 

 to about 2 per cent. 



