26 BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. 



(2) The resistance to wear of igneous and metamorphic rocks, con- 

 taining an abundance of quartz, hornblende, augite, epidote, and 

 garnet, is greater than that of similar rocks rich in mica, chlorite, 

 serpentine, and calcite. 



(3) Foliated metamorphic rocks, owing to the parallel arrangement 

 of their mineral constituents, are, as a rule, deficient in toughness, and 

 therefore not well adapted to road construction. 



(4) Sedimentary rocks are usually deficient in wearing prop- 

 erties, except in the case of highly indurated sandstones, containing 

 a moderate amount of siliceous clay, cement, and limestones or dolo- 

 mites rich in quartz and having very little clay. 



(5) Rocks for road making break down under impact into frag- 

 ments, the shape and physical character of which are conditioned by 

 mineral composition and structure. 



(6) The effect of weathering is generally to lower the resistance to 

 wear of road materials, owing to the development of soft, in part 

 colloidal, products of alteration. Where the secondary minerals are 

 harder and more crystalline the wearing properties of the rocks are 

 proportionately increased. 



(7) The cementing value of road materials is conditioned chiefly by 

 the colloidal products of rock decay and increases in a general way 

 proportionately with these products, reaching a maximum in rocks 

 free from quartz. 



(8) The slaking property of rock powders is dependent in the case of 

 siliceous igneous and metamorphic rocks chiefly on the physical char- 

 acter of the primary mineral components, whereas in basic igneous 

 rocks and sandstones it is caused to a large degree by colloidal prod- 

 ucts of rock decomposition. 



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