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ness generally predominate. Intermixed with the pieces of rock there is likely to be clay or other soil, the quantity varying greatly in different deposits and even in various places in the same deposits. Often there are found deposits of material which are by the layman termed gravel, which are really clayey sand or sand containing a few pebbles, but which are of value to the road builder for the sand clay type of surfacing. The term gravel is exceedingly general and unless specifically defined, gives little indication of the exact nature of to which it is applied. TABLE 7 SHOWING CEMENTING PROPERTIES OF SEVERAL SAMPLES OF GRAVEL -----------------+---------------------------- | Cementing Value Per Cent Clay by +---------------+------------ Weight | As Received | Washed -----------------+---------------+------------ 4.4 | 276 | 43 6.4 | 105 | 285 5.1 | 241 | 70 14.5 | 500 | 279 8.5 | 500 | 112 10.1 | 300 | 267 14.8 | 500 | 107 7.5 | 184 | 198 16.5 | 500 | 428 2.0 | 185 | 239 1.5 | 500 | 500 4.5 | 212 | 204 2.5 | 116 | 363 -----------------+---------------+------------ The value of any gravel for road surfacing depends upon the degree to which it possesses the properties of an ideal gravel for road surfacing. Ideal gravel is seldom encountered, but a consideration of its characteristics serves to establish a measure by which to estimate the probable value of any deposit. =The Ideal Road Gravel.=--The ideal road gravel is a mixture of pebbles, sand and earthy material, the pieces varying from coarse to fine in such a manner that when the gravel is compacted into a road surface the spaces between the larger pebbles are filled with the finer material. The pebbles are of a variety of rock that is highly resistant to wear so that the road surface made from the gravel will have the quality of durability. The gravel possesses good cementing properties, insuring that the pieces will hold together in the road surface. The cementing property may be due to the rock powder in the deposit or
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