8 BULLETIN 370, U. S. DEPAETMENT OF AGEICULTUEE. 
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 
usually been found to lie between 2 and 8, with an average of 5; 
toughness between 7 and 26, with an average of 16 ; 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 may 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 flat plates not suitable for road-building purposes. They are 
seldom used in road construction, except perhaps as a filling for sub- 
foundations. They vary greatly 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 
Eural 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. — The rare rood-building rocks. 
Num- 
Name. 
French coefficient 
of wear. 
Toughness. 
Hardness. 
sam- 
ples. 
Ordinary- 
range. 
Aver- 
age. 
Ordinary 
rango. 
Aver- 
age. 
Ordinary 
range." 
Aver- 
age. 
20 
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 
14 
16.6-19.0 
18. 4-19. 3 
17.6-19.5 
17.5 
10 
IS. 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 
Trachyte 
18.1 
19 
18.1 
