2 BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. 



upon their tendency to influence the physical characteristics of the 

 rocks in which they occur. 



It will be found desirable to outline, in advance, the method em- 

 ployed by this office in the analysis and classification of road materials, 

 as well as to review briefly the mineral composition of rocks and the 

 manner in which their more important physical properties have been 

 determined. 



■m 



EXAMINATION AND CLASSIFICATION OF ROCKS FOR ROAD MAKING. 



Upon receipt of the rock sample, which, according to printed in- 

 structions, should weigh not less than 30 pounds and be collected 

 with care to represent as nearly as possible an average of the whole 

 rock outcrop, it is examined in a general way to determine the proper 

 method of analysis. 



Both rocks consisting chiefly of the carbonate minerals (limestone 

 and dolomite) as well as unconsolidated deposits (clay, gravel, sand) 

 are subject to chemical or mechanical analysis when necessary, 

 whereas all other materials are examined microscopically to deter- 

 mine the quantitative proportions of the minerals present in the 

 rock sample. To carry out a quantitative analysis of this kind it is 

 necessary to prepare a thin section of the sample so that its mineral 

 constituents may be readily recognized and 

 measured. Figure 1 represents such a section 

 of rock mounted in Canada balsam. 



The section is prepared by grinding down 

 with emery powder on a revolving iron disk a 

 representative chip from a hand sample of the 

 rock until a smooth, flat surface is obtained. 

 The chip is then firmly attached to a thick glass 

 plate of appropriate size by means of Canada 

 balsam, with the prepared flat surface against the glass, and the 

 grinding continued on the opposite surface until the section ap- 

 proaches the thickness of tissue paper and becomes transparent, 

 when it is transferred to an object glass and attached by means of 

 the balsam, and finally covered with a very thin cover glass. The 

 minerals present in the slide are then identified under the microscope 

 and their relative proportions determined by means of a measuring 

 apparatus devised by L. W. Page. This consists of an ordinary fixed 

 eyepiece having a square field of about 1 centimeter area divided into 

 100 equal parts, as shown in figure 2. 



With the aid of this crossline micrometer, each square of which is 

 1 one-hundredth or 1 per cent of the whole field, the relative pro- 

 portions, expressed in percentage, of the minerals occupying the field 

 can be readily determined by simply noting the number of squares 

 covered by each mineral in turn. The average results of 20 such 



Fig. 1. — Thin section of 

 rock mounted on glass. 



