EARTH, SAND-CLAY, AND GKAVEL ROADS. 47 
(1) Almost in direct proportion to the percentage of pebbles constituting the 
mass. 
(2) In direct proportion to the value as road metal of the rock fragments 
constituting the pebbles. 
(3) In direct proportion to the value as a cementing material under all 
conditions of weather, of the finer particles of earthy matter constituting the 
filler or binder. 
Since Michigan is one of the leading States in mileage of roads 
surfaced with gravel, the experience of the State highway depart- 
ment, as stated in the paragraphs quoted above, should be of consider- 
able value to road officials in other localities, especially where the 
gravel deposits are principally of glacial origin, as is the case in 
Michigan. But in many sections of the United States the gravel 
deposits contain practically no fragments of limestone or other soft 
rock which might pound up under traffic and serve as a binder to 
hold the larger particles together, and under such conditions clay or 
a sand-clay mixture usually must be depended upon to perform this 
office. 
In general, the physical characteristics of gravel which determine 
its suitability for use in surfacing a road are : 
(1) The durability of the pebbles or rock fragments, (2) the 
quality of the binder, (3) the grading of the pebbles, and (4) the 
proportion in which the binder material is present. While the influ- 
ence each of these factors should exert in fixing the selection of gravel 
for a particular road depends upon local conditions and necessarily 
is a question for individual judgment to decide, nevertheless a few 
points in connection with each factor that may aid materially in 
reaching a decision will be summarized in the following paragraphs. 
DUE ABILITY OF PEBBLES. 
The principal qualities which determine the durability of pebbles 
or stone of any kind when placed in a road surface are hardness, 
toughness, and resistance to wear. The extent to which pebbles pos- 
sess these qualities depends very largely on the character of the 
parent stone from which they were originally produced and accord : 
ingly varies over a wide range. Since nearly all gravel deposits 
contain pebbles which have been formed from many different kinds 
of stone, it has not been considered practicable to apply the ordinary 
laboratory tests 1 for determining hardness, toughness, and per cent 
of wear to gravel, and the matter of comparing these qualities, as 
possessed by the pebbles from different gravel deposits, usually de- 
pends on visual inspection. Not infrequently a very casual inspec- 
tion will reveal which deposit, among a great number, contains 
1 U. S. Department of Agriculture Bulletin No. 347, " Methods for the Determination of 
the Physical Properties of Road-Building Rock." 
