6 BULLETIN 370, U. S. DEPARTMENT OF AGRICULTURE. 
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 they 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 specific 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, 
