* " 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. As these marbles occur only in districts better road making materials are likely to be present, they may not be further mentioned, except to say that their use is commendable for foundation layers, where their fair cementation value makes them tolerably fit for service. So long as the bits are kept from the destructive action of the wheels and feet of the carriages and horses, they lend themselves to the road-master's use. Even where a more resisting top covering of ordinary broken stone cannot be provided, a tolerable road can be made of this material, often very cheaply by using the waste from quarries, by covering the surface with a coating of ferruginous matter, such as is afforded by the leaner iron ores, or by using a top coating of gravel." * " If we take the pure dolomites alone, it is clear that those behave best in the attrition test which have a fine-grained, even, granular texture with irregular shaped grains interlocking closely one with another, and with a general absence of porous cavities. A dolo mite is by no means always better than a limestone, but the best type of dolomite will be more resistant than the best limestones, being harder.
"Among the limestones, those stand highest that are composed of a mixed assemblage of small organic remains, notably of foraminifera, and possess at the same time a somewhat bituminous composition (this characteristic is often associated with foraminifera in carboniferous limestone). Crinoidal limestones do not stand so high, evidently on account of the ready cleavage of the particles of calcite, which is not only a soft mineral but has an extremely perfect cleavage, hence it wears rapidly and crumbles easily under repeated small blows; but if the crinoid fragments are small and uniform in size, set in a matrix of fine calcareous matter, the stone may compare well with other limestones. " Sandstone. As a class sandstones are deficient in cementing power and do not stand well in the surface of a road. They have commonly been sweepingly condemned and rejected by road-builders. In some instances, however, sandstones have given good results, and some of them possess fairly good cementing power. The value of a sandstone depends mainly upon the character of the cementing medium; where this is of siliceous character, a high degree of hardness and resistance to wear may result.
* "Considering the next important group of road making rocks, we notice here also a marked coincidence in mineral composition and physical properties. The soft and nonresistant calcareous rocks (limestones, dolomites, and calcareous sandstones), composed largely of calcite and dolomite, are, as would be expected, inferior in hardness, toughness, and wearing qualities to the more siliceous sandstones and cherts." Cheri. Chert is a very hard material and shows good resistance to wear. It is somewhat low in cement ing value, but when carefully used forms a good road material. It is quite variable in character and needs
careful selection. Chert is commonly found in a finely divided condition, and can be used, in many instances, without crushing. It occurs throughout many of the Southern states, where it is found widely distributed and is the only available material for such • work.
* "The low cementing value of chert obtained by laboratory tests is not in every case in accordance with that developed by this rock under traffic. In dis cussing the origin of road material it has been stated that chert or flint belongs to that class of sedimentary rocks whose mineral components have been formed largely by chemical precipitation and were originally of a colloidal or amorphous nature. The highly fractured condition of many cherts is probably due in large measure to shrinkage caused by a decrease in volume in passing from an amorphous to a crystalline state. Although no experiments have as yet been made on the solubility of this material, it seems to the writer very probable that the dissolving action of road waters on finely divided chert dust is relatively high and that the high binding power of some of these rocks is caused by hydrated opaline silica resulting from a decomposition of this kind. The fact that in certain Incalities surface flints are superior to quarry flints for road making is suggestive in this connection." Mr. Lord, in the bulletin already quoted, gives tables showing the average mineral composition and plates showing structure of the various rocks, and indicates that the probable value of a rock for road-building may be inferred from its mineral composition and structure.
"To explain the bearing of mineral composition and structure on the physical properties of rocks, it has been found necessary to define these properties and describe the various methods for testing road materials. The results of these tests have been used in correlating the physical properties of the various rock families with their mineral components, and the following conclusions have been reached: "(I) Igneous and metamorphic rocks, owing to a high degree of crystallization and a preponderance of silicate minerals, offer a greater resistance to abrasion than nearly all varieties of sedimentary rocks.
" (2) The coarse-grained intrusive rocks of the igneous class are harder, but break more readily under impact than the finer-grained volcanic varieties of like mineral composition.
"(3) The deleterious effect of atmospheric weathering on the wearing qualities of rocks has been demonstrated.
"(4) The cementing value of rocks is, to a certain degree, measured by the abundance of secondary minerals resulting frdm xock decay.
"(5) Metamorphic rocks have, as a rule, a low bind ing power, owing to a regeneration of secondary minerals and to the effects of heat and pressure. The foliated types part readily along planes of schistosity and therefore are not well adapted to road con struction.
" (6) The quantitative mineral analysis of rocks serves to a certain extent as a measure of their useful properties for road construction. "