PHYSICAL PROPERTIES OF ROAD MATERIALS. 



21 



and nonfoliated CNos. 9-15) metamorphic rocks, given in Table 7 

 contain relatively low percentages of secondary minerals, and have 

 not been further subdivided, with reference to these, except in the 

 case of quartzites. 



In comparing these rocks it will be noted that the foliated types, with 

 the exception of more compact hornblendic varieties (Nos. 7 and 8), 

 are lower in hardness, toughness, and indicated wearing properties 

 than nonfoliated types excepting the highly micaceous slates (No. 9) 

 and crystalline marbles (No. 15). It will be observed, furthermore, 

 that the cementing value of metamorphic rocks is relatively low in 



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 CEMEA/r/MG VALUE ' -A/P OF BLOWS 

 Fig. 3. — Effects of alteration products on cementing properties of rock powders. 



accordance with low percentages of kaolin and secondary chlorite, 

 reaching a maximum of 49 in amphibolite and 56 in slate (Nos. 8 and 

 9), with 13.5 per cent and 9.3 per cent, respectively, of secondary 

 minerals, and a minimum of 17 in quartzites having each about 1.5 

 per cent decomposition products (Nos. 10 and 12). The very slight 

 effect of even appreciable quantities of secondary minerals (No. 11) 

 on the cementing properties of these highly quartzose rocks is similar 

 to that found in acid granite (see p. 16), and may be attributed in 

 both cases to the physical character of the quartz fragments com- 

 bined with insufficient colloidal material. It will be of interest in 

 this connection to compare these altered quartzites (No. 11), having 



