PHYSICAL TESTS OF ROAD-BUILDlI^G ROCK. 7 



MARBLE AND QUARTZITE. 



Marble and quartzite are the two families of nonfoliated meta- 

 morphic rocks corresponding to limestone and sandstone, respec- 

 tively. While in some respects it is convenient to consider marble 

 with the limestone and dolomite group, it will be seen from the chart 

 that the average toughness of marble, about 5, is lower, and that the 

 average hardness, which is less than 14, is also somewhat lower. 

 Marbles usually show good cementing value tests with about the same 

 range as the limestones and dolomites. For those samples tested, the 

 specific gravity ordinarily falls between 2.7 and 2.9 and the weight 

 per cubic foot averages 173 pounds, which is somewhat higher than 

 the average for either limestone or dolomite. As would therefore be 

 expected, the maximum absorption is less, being under 2.5 per cent. 



Quartzites show an average toughness of 15, as compared with 10 

 for the sandstones. The coefficient of hardness is also higher and 

 for the samples tested shows a much smaller range of values than for 

 the sandstones. The quartzites invariably show a low cementing 

 value. Their specific gravity from tests made usually lies between 

 2.6 and 2.8 and their average weight per cubic foot is about 167 

 pounds. Their water absorption runs from a few hundredths of 



1 per cent to nearly 3 per cent. 



GNEISS AND SCHIST. 



Both gneiss and schist belong to the foliated metamorphic type 

 of rocks. The former is in reality a metamorphosed granite and 

 therefore shows physical properties similar to the granites. The 

 average French coefficient of wear for the gneiss samples is about 9, 

 being somewhat lower than for the granites, while their average 

 hardness and toughness is about the same. Their specific gravity, 

 weight per cubic foot, and absorption are approximately the same 

 as for granite. 



The schists show an average French coefficient of wear of about 12. 

 Their average hardness is about 17.5 and their toughness averages 11, 

 the latter being higher than for gneiss. It should be noted, however, 

 that the toughness test for both gneiss and schist is made perpen- 

 dicular to the plane of foliation. If taken horizontal to the plane 

 of foliation much lower results would be obtained, as failure would 

 then occur along these natural lines of cleavage. The specific gravity 

 of schists usually lies between 2.65 and 2.90 and the average weight 

 per cubic foot is about 181 pounds. Water absorption is seldom over 



2 per cent for this family. 



With the exception of the highly altered varieties, both gneisses 

 Sij\d schists show a rather low cementing value. 



