PHYSICAL PR0PERT1KS ()!•' ItOAD MATERIALS. 19 



microscopically, it was found necessary to subject limestones and 

 dolomites to chemical analysis by means of which quartz, kaolin, 

 and limonite could be separated quantitatively from the carbonate 

 minerals. The results of the physical tests and mineral analysis of 

 sedimentary rocks are given in Table 6. It will be seen that lime- 

 stones and dolomites have been separated into groups having about 

 equal quantities of quartz and secondary constituents, while sand- 

 stones have been divided into families representing averages of 

 samples containing from 1 to 5 per cent, from 5 to 10 per cent, 

 from 10 to 20 per cent, from 20 to 30 per cent, from 30 to 40 per 

 cent, and above 40 per cent, of secondary minerals. 



Comparing these results it will be observed in the first place that 

 the average toughness and indicated wearing properties of calcareous 

 rocks (Nos. 1-4) are as a rule inferior to sandstones (Nos. 5-10) 

 while their cementing values are about the same. The exception- 

 ally high percentage of wear found for chert (No. 11) may be explained 

 by the fractured condition of this material caused by contraction on 

 consolidation of the colloidal silica. 1 



It will be noticed, furthermore, that varieties of limestone and 

 dolomite containing an appreciable amount of quartz with some 

 kaolin or clay (Nos. 2 and 4) are tougher and have lower percentages 

 of wear and higher cementing values than samples deficient in these 

 minerals (Nos. 1 and 3). It would appear also that limestones, owing 

 to their greater solubility in water, cement better than dolomites. 



Turning to the sandstones it will be observed that their toughness 

 and cementing values increase proportionately with secondary 

 components up to rocks averaging about 15 per cent (No. 7) and 

 that beyond this point a general falling off in hardness and toughness 

 takes place while the cementing value remains practically constant 

 or decreases slightly with proportionately increased amounts of 

 calcite (Nos. 9 and 10). It should be stated, however, that not only 

 the quantity and quality of cementing material present in this class 

 of road material but also the fineness of grain and density of struc- 

 ture influences the physical properties to a very marked degree. 2 

 Therefore it may be said that compact, fine-grained sandstones, 

 having a moderate amount of siliceous clay cement (No. 7), produce 

 harder and tougher material than rocks with deficient binder (Nos. 

 5 and 6) or such containing a large excess of calcareous clay cement 

 (No. 10). 



METAMOEPHIC ROCKS. 



The chief result of mineral alteration by metamorphic agencies is 

 the development of highly crystalline rocks having characteristic 

 structures, and composed of minerals very resistant to atmospheric 

 weathering. For this reason the various types of foliated (Nos. 1-8) 



i Office of Public Roads, Bui. 37, p. 27. 



2 J. Hirschwald. Handbuch der bautechnischen Gesteinspruefung, 1912, pp. 360-4S4. 



