Characteristics of Different Gravels

Lexington.This gravel is used in and around Lexington, McLean County, Ill., for country highways with entire satisfac tion. Notice that the clay is equal to only one seventh of the voids. Nearly all of the 21 per cent of ferruginous limestone consists of thin chips which are easily crushed by the traffic. Some binder is probably obtained from the 58 per cent of silicious lime stone. The per cent of crystalline rocks present is very small, and can not materially affect the quality of the gravel. The amount of quartz is less than in the preceding gravels, and is an unim portallt element.

Rockford.This gravel has given satisfactory service in Rock ford, Winnebago County, Ill., probably under more exacting con ditions than any of the preceding. This is considerably the coarsest gravel in Table 17. Notice that this gravel contains, roughly speaking, only about one tenth enough clay to fill the voids. The chief source of binder is the limestone which ex ists in the form of pebbles, but contains no considerable amount of iron or silica. The basic crystalline rocks by decomposing may furnish a little binder; but as they are round hard pebbles, not easily crushed, the binder derived from this source can be of no • practical importance. A very little cementing material may be derived from the iron conglomerate and also from the limestone and quartz conglomerate.

Peekskill.This gravel is from Roa Hook, a " point " in the Hudson river near Peekskill, Ni Y., and is much used in and around New York City, where it is considered one-of the best road gravels. Notice that the clay is less than one thirtieth of the vol ume of the voids. Considerable binding material is doubtless derived from the ferruginous limestone, which contains a com paratively small per cent of iron. The iron in the ferruginous sandstone is too small in amount to be appreciable. Some binder is doubtless derived from the metamorphosed rocks containing iron, silica, and mica. Notice that there are nearly 30 per cent of crystalline rocks, which upon being finely pulverized will fur nish an excellent cementing material, particularly after being decomposed. This gravel requires considerable rolling with a heavy roller to crush the several ingredients and liberal sprinkling to work the pulverized material into the voids, before the mass binds. All the other gravels bind and make fair roads under ordi nary traffic.

Buck Hill.

This gravel was obtained from the Buck Hill pit at Tuckahoe, N. J. It was recommended as a representative gravel by Hon. Henry I. Budd, State Commissioner of Public Roads of New Jersey. This gravel consists practically of clay and partially rounded quartz pebbles. The metamorphcsed

rock is angular and friable. The clay is probably enough to fill the voids when the gravel has been compacted by traffic. This is the first of the samples in which the iron contaired in the clay is appreciable, and the iron doubtless has an important part in binding the road. This gravel is used for road building with out rolling.

Rock Hill.

This sample was obtained from the Rock Hill pit at Tuckahoe, N. J., and is substantially the same as No. 6 above, except in having a greater per cent of voids and in contain ing some sandstone which crushes easily and materially reduces the voids of the gravel after it has been compacted in the road. It is said that the best results are obtained by mixing this and the preceding gravel half and half.

Shark River.

This gravel was obtained from the Manasquan Gravel Co. of Asbury Park, N. J., and is much used in Southern New Jersey and around New York City. It consists wholly of clay and small rounded pebbles of pure white quartz, and conse quently the only binding material is the clay and the 2 per cent of iron contained in it.

Oaktown.This is a gravel obtained from the Wabash river, a few miles above Vincennes, Ind., by dredging, which has been used on the roads entering Oaktown, Knox Co., Ind. There is very little clay in this gravel,—only 7.1 per cent, if the shale be considered as clay, as it is practically. The chief source of binding material is the 18.9 per cent of carbonate of lime, much of which is in the form of flat chips. The metamorphic rocks are also in thin chips, and are easily pulverized. The crystalline rocks and the quartz are comparatively rough and angular. In service the limestone pebbles grind up under traffic, and the road becomes hard and firm, and is not much affected by freezing and thawing.

Shaker Prairie.

This gravel is found in a pit on Shaker Prairie, west of Oaktown, Knox Co., Ind., and consolidates under traffic much more quickly than the preceding, the two being used side by side. This gravel contains only a comparatively small amount of fine sand, being in this respect about on a par with the Peekskill gravel—see No. 5, Table 17. It contains a compara tively large amount of clay, being in this respect similar to the New Jersey gravels—No. 7, 8, and 9 in Table 17 and 18. This gravel has more iron in the clay than any of the samples except the Tuckahoe gravels—No. 6 and 7. The limestone is in compara tively large rounded pebbles, and not specially easily crushed under traffic. The road is bound almost wholly by the clay and the iron in it and by the pulverized limestone.