6 



BULLETIN 348, TJ. S. DEPARTMENT OF AGRICULTURE. 



ment, through metamorphic agencies, of muscovite, chlorite, epidote, 

 hornblende, garnet, etc. (PL VII, figs. 5, 7, 9, 10, and 12). . Thus it 

 has been found that granite passes over into gneiss and micaceous 

 schist (PI. 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 



Calcite 



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. 



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. 



