PHYSICAL PROPERTIES OF ROAD MATERIALS. 13 
The before-mentioned differences in physical properties of rocks for 
road making find expression also in the shapes of their smaller frag- 
ments as shown in Plate VIII. 
It will be observed that the screenings from coarse-grained plutonic 
rocks after removal of the fine dust (figs. 1 and 3) are more rectangular 
in shape than those from finer-grained volcanics of similar composi- 
tion which appear splintery and wedge-shaped (figs. 2 and 4). These 
characteristic shapes may be ascribed, as previously indicated, to the 
cubical cleavage of coarse, granular feldspar in the plutonic rocks and 
to the denser structures developed in the finer-grained rock types. 
The peculiar rounded shape of more loosely textured sandstones and 
the flat, lath-like appearance of fragments from schistose rocks (figs. 
5 and 6) are noteworthy and may be associated directly with the 
mode of origin and mineral composition of this material. (See pp. 4-6.) 
Regarding the binding effect of these rock types on the road it can 
be stated in general that the tough wedge-shaped fragments from 
fine-grained igneous rocks are inclined to produce a firmer and more 
permanent bond with the coarser road stones when properly com- 
pacted than is obtained with the more cubical or rounded screenings 
from granite and sandstone or with the lath-shaped schists and 
gneisses. This interlocking of the coarser rock fragments under 
the roller may be termed the mechanical bond of the road surface as 
distinguished from the cementing bond or binding effect produced 
by the finer rock dust when sufficiently moistened and compressed. 
In discussing the mineral composition of road materials this cementing 
property has been attributed largely to the peculiar quality of certain 
amorphous compounds closely related to kaolin and chlorite in 
composition. It will now be of interest to compare varying propor- 
tions of these compounds as well as other secondary minerals found 
in the more common rock types and note their effect on the cement- 
ing value and other physical properties of the rocks. 
THE EFFECT OF SECONDARY MINERALS ON THE PHYSICAL PROPERTIES 
OF ROCKS FOR ROAD BUILDING. 
The majority of rocks used for road building have been obtained 
from near the earth's surface or within the zone of atmospheric 
weathering, 1 and the extent of alteration varies greatly, depending 
upon composition, rock structure, and character and time of opera- 
tion of the agencies involved. The resulting minerals are also 
manifold, being either softer products of hydration and oxidation, 
such as kaolin, chlorite, limonite, calcite, and opaline silica, or the 
harder silicates, epidote and zeolite, accompanied in some cases by 
secondary hornblende (actinolite) and quartz resulting from deeper- 
1 G. P. Merrill, Rocks, Rock-Weathering and Soils, 1906, p. 152. 
