PHYSICAL PROPERTIES OF ROAD MATERIALS. 
measurements taken in various parts of the section will, under 
ordinary conditions, give the volumetric proportions of the minerals 
in the rock itself. If desired, the chemical composition of the rock 
may be approximately obtained by multiplying the volumetric per- 
centages of all the minerals present by their specific gravities and 
reducing the whole to a basis of 100, and then multiplying the values 
obtained by the percentages of the chemical components, reckoned 
as oxides, belonging to each mineral. 1 
The average mineral composition and physical properties of all 
rock samples analyzed and tested in this office up to January 1, 1914, 
are given in Table 1. 
The material has been arranged in groups following the general 
classification adopted by this office, which is as follows : 
Table 2. — General classification of rods. 
Class. 
Type. 
Family. 
1. Intrusive (plutonic). . 
2. Extrusive (volcanic).. 
ia. Granite. 
b. Syenite. 
]c. Diorite. 
d. Gabbro. 
(e. Peridotite. 
fa. Rhyolite. 
lb. Trachyte, 
ic. Andesite. 
(d. Basalt and diabase. 
]a. Limestone. 
\b. Dolomite. 
ra. Shale. 
lb. Sandstone. 
Lc. Chert (flint), 
fa. Gneiss, 
{b. Schist. 
|c. Amphibolite. 
(a. Slate, 
lb. Quartzite. 
ic. Eclogite. 
[d. Marble. 
II. Sedimentary 
III. Metamorphic.... 
2. Siliceous 
1. Foliated 
2. Nonfoliated 
Referring to Table No. 1, Nos. 1 to 7 embrace plutonic igneous 
rocks; 8 to 14, volcanic igneous rocks; 15 to 20, sedimentary rocks; 
and 21 to 35, metamorphic rocks or crystalline schists. The aver- 
age percentages of essential minerals characterizing each rock variety 
are printed in bold type and the minerals themselves placed in the 
table approximately in the order of their importance as rock-forming 
constituents. 2 The relative value of these various rock types from a 
road-making standpoint may be best understood after a brief discus- 
sion of their mode of origin and mineral characteristics. Under igne- 
ous rocks (Nos. 1 to 14) are included all varieties of road material 
that have solidified from a state of liquid fusion, the structure or 
manner of mineral aggregation being influenced largely by the rate 
of cooling of the fused rock magma. Thus, intrusive or plutonic 
i Bulletin 37, Office of Public Roads, pp. 10-11. 
2 The mineral composition of limestone, dolomite, and marble has been obtained from chemical analyses 
of about 100 samples. 
