9°6 Journal of Agricultural Research voi. v, no. ig 
corresponding limiting values for hardness at io, 14, and 17. In other 
words, after making all allowances for variations due to local conditions, 
it may be fairly assumed that stone for use under light, horse-drawn, 
steel-tired vehicles should show a toughness of from 5 to 9 and a hardness 
of from 10 to 17; for moderate traffic a toughness of from 10 to 18 
and a hardness of over 14, and for heavy traffic a toughness of 19 or 
over and a hardness of 17 or over. The terms “light,” “moderate,” 
and “heavy” in this connection refer to the total volume of traffic upon 
the road, calling, say, under 100 teams a day “light,” 100 to 250 “mod¬ 
erate,” and over 250 “heavy.” 
Practically all the values of hardness shown in figure 1 are above the 
various lower limits set by the best water-bound macadam-road practice. 
For light-traffic conditions, 94 per cent of all the samples tested have a 
hardness of more than 10; for moderate traffic, 95 per cent have a 
hardness of more than 14; and for heavy traffic, 94 per cent have a hard¬ 
ness of more than 17. 
In other words, if it be assumed that the curve (fig. 1) represents a fair 
average of all available types of road-building rock, it would seem that 
a determination of the toughness of any particular sample of rock shows, 
for all practical purposes at least, whether it is hard enough to be satis¬ 
factorily used in construction. 
If the curve be referred to again, it will be seen that a large number of 
hardness tests appear above the upper limit of 17 set for light-traffic 
conditions. Although on its face this would indicate that a determina¬ 
tion of the hardness is necessary in this instance, reference to test records 
show that by far the greatest number of these tests (about 75 per cent) 
are on granites, quartzites, and hard sandstones, which are unsuited for 
use in the wearing course of water-bound macadam roads, owing to their 
lack of binding power, as shown by actual test. 
Finally, the results of 2,500 individual routine tests made by the Office 
of Public Roads and Rural Engineering show that for practical routine 
work the hardness test adds nothing to our knowledge of the value of 
any particular rock sample for use in water-bound macadam-road 
construction over that obtained from the toughness test. 
While the binding or cementing value of a rock is a most important 
consideration from the standpoint of ordinary macadam construction, the 
same is not true of broken-stone roads which are surface treated or con¬ 
structed with an adhesive bituminous material. The hardness of the 
rock is also of relatively less importance, owing to the fact that the fine 
mineral particles produced by the abrasion of traffic combine or should 
combine with the bituminous material to form a mastic which is held in 
place and protects the underlying rock from abrasion so long as by proper 
maintenance it is kept intact. The toughness of the rock, however, is 
of more importance, as the shock of impact is to a considerable extent 
transmitted through the seal coat and may cause the underlying fragments 
