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Characteristics and Distribution of Road-Build Ing Rocks

road, granite, stones, stone, cementing, wear and rock

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CHARACTERISTICS AND DISTRIBUTION OF ROAD-BUILD ING ROCKS. Experience in France and Massachusetts has shown that the results of the impact and abrasion test agree reasonably well with the relative values of the stones as determined by actual wear in the road. The different stones are arranged in Table 17 sub stantially in the order of their wear in the impact and abrasion test, and consequently this order represents approximately the relative value of the several stones for road-building purposes. However there are some exceptions to this order. Where the traffic is light a hard stone may not furnish enough dust to re place that blown away by the wind and washed away by the water, and also to bind the surface; and in such case a softer stone or one with a higher cementing power may be preferable. A suitable road stone should be soft enough to grind to dust only slowly under the traffic, and this dust should have a high cementing power, and the separate fragments of the stone should have strength enough to resist the crushing action of the wheels.

The characteristics and distribution of the road-building ma terials which are extensively available will now be briefly con sidered.

Trap. This is a popular term applied to any dark-colored, massive, igneous rock. Trap is very compact and elastic, has a high resistance to crushing without being brittle, and its dust has the cementing power in a high degree. The different traps are not uniformly desirable, but nearly all of them are better than the best of other rocks; and therefore the traps may clearly be placed first in order of utility among road-building stones.

Trappean rocks are plentiful in the greater part of New England, in the upland districts of New Jersey and the neigh boring portions of Maryland and Pennsylvania, in the Blue Ridge mountains between the Potomac and the James rivers, in the basin of Lake Superior, particularly in the northern peninsula of Michigan, and in the Rocky mountains. They are also found to a limited extent in southern Missouri, in Arkansas, and in central Texas.* In America the trap rocks are not as plentiful nor as evenly dis tributed as in Europe.

The traps of Table 19, page 186, include eight samples from New Jersey, two from New York, two from Connecticut, and one from Rhode Island, the remainder, forty-six, being from Massachusetts. Granite. Next in value to the trappean rocks are those commonly called granites. These are massive granular rocks

composed essentially of quartz and feldspar, but almost always containing mica, hornblende, and other components. An tial feature of granite is an evenly granular structure coarse enough to be distinctly visible to the naked eye. Granites vary widely, but as a rule they are inferior road material owing to the ness of both the quartz and the feldspar, and also to their coarsely granular structure. If the quartz is absent. the rock is what is technically called syenite, the best for road metal of the so-called granites. When granite is free from mica, it offers great resistance to wear. If the granitic rock has a pronounced fibrous or hairy arrangement of its mineral constituents, the rock is termed gneiss, most samples of which are very inferior road materials.

The feldspar of granite is readily decomposed, producing sand and clay; and when this change has gone on to a considerable degree, the stone should be discarded as unfit for road material, since it readily crumbles to sand and clay under the action of frost and traffic, and the wind sweeps away the fine material in dry weather, and in rainy times the road is in a muddy state. For this reason , granite is more satisfactory in southern than in northern localities.

"The distribution of the granite rocks is unfortunately in a general way the same as that of the trappean rocks (§ 289). Between the traps and the granites about one third of the area of the Unite I States is fairly well provided with road-making stones." * Limestone. The limestones are usually deficient in hard ness and toughness, but possess cementing power in a fair degree (see Tables 19 and 20). Limestones where found in thin layers with little sign of crystallization, particularly where they contain a small an count of clay,—say, not more than twenty-five per cent, —often afford tolerable road stones. In proportion as limestone becOmes crystalline, i, e., takes on the character of marble, its value in road-making diminishes, for the reason that the crystalline structure in most cases so far weakens the mass that it is apt readily to pass into the state of powder. Marble has a high per cent of wear and a h 4w cementing power—see Table 19. Marbles occur only in districts where better road-making materials are likely to be present.

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