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, 1G06, p. 152. 



