Page:Popular Science Monthly Volume 51.djvu/90

82 example, quartz or quartzite has a hardness of seven in a scale of ten, and for this quality alone it is best suited for road building of any rock of common occurrence; but it does not possess any cementing power or elasticity whatever, and is therefore of little use for Macadam work. As a result of long trial on roads in England and on the Continent, it is found that the stone best suited for road metal must possess toughness and cementing qualities and as great a resistance to abrasion as is possible in a stone having the first two properties. It is an important fact that experimental investigation in the laboratory has pointed scientifically to the same conclusions that have been obtained from the severe test of long experience in actual use.

The most important road stones are known under the common names of "trap" or "dike stones." They are usually of a dark-green color, are fine-grained, and are composed essentially of the minerals pyroxene or hornblende and feldspar, the individual minerals often not being visible to the unaided eye. Geologically, they are rocks that have been forced up through fissures in the earth's crust from great depths, where they existed in a melted condition. Rocks of this kind are very numerous in eastern Massachusetts and generally throughout the old mountain ranges of the United States. The road engineer, however, has other materials besides quarry stone, which, though not possessing so many good qualities, nevertheless make excellent road metal under proper conditions. Among these may be mentioned the blue glacial gravels, kame gravels, beach pebbles, and field stones.

Another rock in common use in various parts of New England is granite (a mixture of the minerals quartz and feldspar) and the allied rock, gneiss. Both these rocks are normally coarse-grained, possessing a hardness, as measured by that of their component minerals, a little under seven. In its use as broken stone granite has certain advantages over quartz alone, in that the feldspar, when pulverized or decomposed by the action of the weather, has considerable cementing value; but the decomposition of the feldspar liberates the quartz, and the physical differences in the matter of hardness, cleavage, etc., between the quartz and the feldspar promote differential wear of the stone as well as other defects. Granite, however, is an important road stone, and is far superior to such rocks as limestones, slates, or marbles, which, owing to their softness, are rapidly worn out.

The production of broken stone has now assumed such importance that several concerns in Massachusetts are making a regular business of furnishing all sizes to the State or municipalities. Broken stone, as a commercial commodity, is now sold on the cars at about one dollar to one dollar and seventy cents per ton for the best quality of trap.