PHYSICAL TESTS OF ROAD-BUILDING 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 
and schists show a rather low cementing value. 
