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 " 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 greathy 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 partialty 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 which do not carry sufficient 

 binder, clay usually is the material added to correct the deficiency. 



