26 BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. 
(2) The resistance to wear of igneous and met amorphic 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 amomit 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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