2 BULLETIN 348, U. S. DEPARTMENT OF AGEICULTUEE. 
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. 
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. 
fig. i.— Thin section of The chip is then firmly attached to a thick glass 
rock mounted on 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 
