PHYSICAL PROPERTIES OF ROAD MATERIALS. 25 



It has been demonstrated, furthermore, that the shape and physical 

 character of the smaller rock fragments used in road making are 

 largely dependent upon structure and mineral composition, coarse- 

 grained feldspathic rocks breaking down readily into rectangular 

 fragments owing to feldspar cleavage, whereas the finer-grained trap 

 screenings are more wedge-shaped and tougher. The chips broken 

 from loosely compacted sandstones and schists are rounded in shape, 

 approaching somewhat the outline of abraded quartz grains, or they 

 appear flat and lath-shaped in accordance with the foliated structure 

 of the latter rocks. The effect of weathering on the physical prop- 

 erties of rocks is clearly shown and varies with the structural char- 

 acter and composition of the rock and the physical properties and 

 relative abundance of alteration products. Thus the indicated wear- 

 ing properties of igneous and metamorphic rocks are, in general, 

 benefited by hard secondary epidote, zeolite, and hornblende, while 

 an abundance of softer alteration products such as chlorite, serpentine, 

 and kaolin lowers the indicated resistance to wear. 



In sedimentary limestones and dolomites it has been found that 

 quartz accompanied by a little kaolin has greatly increased their 

 toughness and resistance to abrasion, whereas compact sandstones 

 having about 15 per cent of secondary constituents are among the 

 most resistant of sedimentary rocks. The effects of weathering have 

 been found most pronounced in regard to the cementing property of 

 road materials, which is seen to vary directly, but not in the same 

 ratio, with the amount of soft secondary minerals, in part amorphous. 

 Where these materials are replaced, even to a limited extent, by 

 harder, more crystalline products of alteration the binding properties 

 are proportionately lowered. These relations have been plotted in 

 curves for the common crystalline rock types and gravels. 



Finally, the slaking properties of rock powders have been investi- 

 gated with the result that acid crystalline rocks are found to slake 

 rapidly excepting samples that contain relatively low average per- 

 centages of mica and quartz and abundant hornblende and epidote, 

 while the slaking qualities of basic igneous rocks and sandstones are 

 dependent mainly on the relative abundance of colloidal products of 

 rock weathering, such as kaolin, limonite, and chlorite. 



The results obtained from the present investigations indicate the 

 following : 



(1) Igneous and nonfoliated metamorphic rocks, owing to a pre- 

 ponderance of hard silicate minerals combined with greater uniformity 

 in structure, are more durable than other road-making materials, 

 finer-grained varieties offering greater resistance to abrasion than 

 coarse-grained types. 



