6 
BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. 
rnent, through metamorphic agencies, of muscovite, chlorite, epidote, 
hornblende, garnet, etc. (PI. VII, figs. 5, 7, 9, 10, and 12). Thus it 
has been found that granite passes over into gneiss and micaceous 
schist (PL VI), and from the more basic gabbro, diorite, etc., are 
developed amphibolite, chlorite schist, eclogite, etc., while sandstone 
and limestone give place, respectively, to quartzite (PI. V, fig. 1) 
and marble. 
The chief interest attached to the study of these minerals is to 
determine in what manner they affect the road-making qualities of 
rocks, and it will be, therefore, of interest to discuss somewhat in 
detail their more important physical properties. 
In Table 3 are given the essential mineral constituents of rocks 
for road making, with their general chemical composition and volu- 
metric percentages as determined from all samples thus far analyzed. 
Table 3. — Mineral constituents of rocks for road making. 
Primary minerals. 
Secondary minerals. 
Name. 
Chemical composition. 
Volu- 
metric 
per- 
centages. 
Name. 
Chemical composition. 
Volu- 
metric 
per- 
centages. 
Quartz 
Plagioclase... 
Orthoclase... 
Augite 
Silica 
17.8 
9.6 
8.8 
6.0 
3.1 
2.4 
1.7 
1.5 
1.2 
0.2 
Caleite 
Dolomite 
Kaolin 
Chlorite 
Epidote 
Limonite 
Serpentine . . 
Talc: 
26.5 
Silicate of alumina, lime, 
and soda. 
Silicate of alumina and 
potash. 
Silicate of lime, magnesia, 
iron, and alumina. 
.... do 
Carbonate of lime and mag- 
nesia. 
Hydrous silicate of alumi- 
na. 
Hydrous silicate of mag- 
nesia, iron, and alumina. 
Hydrous silicate of lime, 
iron, and alumina. 
Hydrated oxide of iron 
Hydrous silicate of iron 
and magnesia. 
Hydrous silicate of mag- 
nesia. 
Hydrous silicate of alumi- 
na, lime, and soda. 
10.4 
4.2 
2. 7 
1.7 
Muscovite 
Biotite 
Hydrous silicate of alumi- 
na and potash. 
Hydrous silicate of alumi- 
na, iron, magnesia, and 
potash. 
Variable 
1.2 
0.1 
0.1 
Magnetite 
Magnetic oxide of iron 
Silicate of alumina, iron, 
and lime. 
Zeolite 
Opal 
0.1 
0.1 
It will be noted that these constituents have been divided into two 
groups — primary minerals, or such that constitute the original com- 
ponents of igneous rocks and form the essential part of metamorphic 
schists, and secondary minerals that have been derived from the for- 
mer through subsequent processes of alteration. 
In the case of certain volcanic rock magmas (basalt, rhyolite, ande- 
site), where the rate of cooling has been very rapid, a glass base (rock 
glass) is frequently left after crystallization has ceased. This glass 
is extremely brittle and when present in appreciable quantities has a 
tendency to lower the indicated wearing properties of the rock. 
