=Natural Gravel.=--Gravel is the name given to a material consisting of a mixture of more or less rounded stones, sand and earthy material, which is found in natural deposits. These deposits exist in almost every part of North America, being especially numerous in the glaciated areas, but by no means confined to them. Gravel deposits consist of pieces of rock varying in size from those of a cubic yard or more in volume to the finest stone dust, but with pieces ranging in size from that which will pass a 3-inch ring down to fine sand predominating. The larger pieces are usually more or less rounded and the finer particles may be rounded or may be angular. Many varieties of rocks are to be found among the gravel pebbles, but the rocks of igneous origin and possessing a considerable degree of hardness 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.