48 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 
the largest percentage of hard durable pebbles. There are compara- 
tively few cases where this point can not be determined with suffi- 
cient accuracy by sorting out the pebbles contained in representative 
samples from the different deposits, testing the various kinds with a 
hand hammer, and determining the relative proportions in which 
the more durable materials are present in the sample by means of a 
weighing device. 
Ordinarily the most durable pebbles which occur in gravel deposits 
are those composed of either quartzite or chert, though many de- 
posits contain fragments of limestone or trap rock, either of which 
may make an excellent road-surfacing material. For example, the 
so-called u blue gravel " which occurs in certain sections of the coun- 
try usually is composed largely of trap-rock fragments and. generally 
ranks high as a road material. "While limestone pebbles are less 
resistant to wear than those of trap rock, they possess the advantage 
of a relatively high cementing value and their presence in gravel 
may aid greatly in securing a well-bonded surface without an excess 
of clay. 
The pebbles which are least durable but very prevalent in gravel 
deposits are those composed of sandstone. As a rule, such pebbles are 
lacking greatly in toughness and will shatter under traffic. Gravel 
deposits also may contain a high percentage of partially disinte- 
grated pebbles which are even less durable than sandstone. " Bastard 
granite"' is a common example of partially disintegrated rock, and 
not infrequently it happens that gravel deposits contain a high per- 
centage of this material. Partially disintegrated chert is found occa- 
sionally in gravels, but as such material possesses a relatively high 
cementing value, its presence in small proportion may be an advantage 
rather than a detriment to gravel deposits. 
THE BINDER. 
No matter how durable may be the pebbles contained in a given 
gravel deposit, they can not be used successfully in a road surface 
unless they can be well bonded together so as to present a combined 
resistance to the disturbing action of traffic. To accomplish this 
bond requires that the gravel contain some cementing or binding 
agent such as iron oxide, carbonate of lime, or clay. As already ex- 
plained, certain pebbles, such as those composed of limestone, possess 
the property of becoming firmly bonded together by virtue of their 
own cementing value. 
The principal cementing agent, or binder, present in most gravel 
deposits is clay, and in case of deposits ^hich«do not carry sufficient 
binder, clay usually is the material added to correct the deficiency. 
