EARTH; SAND-CLAY, AND GRAVEL 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. 



DURABILITY 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." 



