2 BULLETIN 670, U. S. DEPARTMENT OF AGRICULTURE. 
The percentage variation in strength of 153 granites and gneisses 
and 156 limestones and dolomites, the total number of these types 
of rock tested up to January 1, 1918, is shown graphically in Fig- 
ure 1. In this chart the percentage of total samples tested having 
various values for crushing strength are plotted as indicated. As 
an example, the chart shows that 7 per cent of all granites and 
gneisses tested have a crushing strength of 21,000 pounds per square 
inch. The sum of all per cents to the left of the 21,000-pound line 
totals 50; that is, 50 per cent of all granites and gneisses tested have 
a crushing strength of less than 21,000 pounds. On the other hand, 
7 per cent of all the granites and gneisses show a crushing strength of 
20,000 pounds per square inch, and by summing up all the per cents 
to the right of the 20,000-pound line, it will be seen that 50 per cent 
of the samples tested have a crushing strength greater than 20,000 
pounds per square inch. In other words, the average crushing 
strength of this type of material lies between 20,000 and 21,000 
pounds per square inch. By a similar interpretation, the average 
crushing strength of limestones and dolomites lies between 18,000 
and 19,000 pounds per square inch. Further study of the chart 
shows that the crushing strength of limestones and dolomites closely 
approaches that of granites and gneisses. 
INTERPRETATION OF RESULTS OF PHYSICAL TESTS. 
To interpret the results of the physical tests made of road-build- 
ing rock, the office has adopted a table of general limiting test values 
for broken stone for the various types of road construction which 
is printed on the back of the form for reporting tests. For general 
reference, these Hmiting values, together with comments upon limits 
shown, are given in Table IV. By comparing the results of tests 
on a sample of rock with the limits shown in the table, a general 
idea of the types of road construction for which it is best suited 
may be obtained. Table III contains the total number of rock 
samples received from the various States which have been tested 
up to January 1, 1918. 
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CRUSHING STREN6TH — u&S. PER SQ. IN. 
Fig. 1.— Variations in the crushing strength of rock. 
