PHYSICAL PEOPEBTIES OP EOAD MATEEIALS. 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. 1 1 ) 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). 
METAMORPHIC 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, Bill. 37, p. 27. 
2 J. Hirschwald. Handbueh der bautechnischen Gesteinspruefung, 1912, pp. 360-484. 
