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 he 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 rocks. 



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, 

 ia. Limestone. 

 \b. Dolomite. 

 ra. Shale. 

 Jb. Sandstone. 

 Ic. Chert (flint), 

 fa. Gneiss. 



b. Schist, 

 [e. Amphibolite. 

 fa. Slate, 

 lb. Quartzite. 

 Ic. Eclogite. 

 (d. Marble. 



II. Sedimentary 



III. Metamorphic 





* 



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. 



