Classification. The rocks used for road-building differ widely in their mineral characters. The classifi cation' shown in the following table is proposed by Mr. Edwin C. E. Lord.* "All rocks of the igneous class are presumed to have solidified from a molten state, either upon reaching the earth's surface or at varying depths below it. The physical conditions, such as heat and pressure, under which the molten rock magma consolidated, as well as its chemical composition and the presence of included vapors, are the chief features influencing the structure. Thus, we find the deep-seated, plutonic rocks coarsely crystalline with mineral constituents well defined, as in case of granite rocks, indicating a single, prolonged period of development, whereas the members of the extrusive, or volcanic, types, solidifying more rapidly at the surface, are either fine grained or frequently glassy and vesicular, or show porphyritic structure. This structure is produced by the development of large crystals in a more or less dense and fine-grained ground mass, and is caused generally by a recurrence of mineral growth during the effusive period of magnetic consoli dation. Rocks of this kind, exhibiting a more or less spotted appearance, are commonly described as por phyries, regardless of mineral composition, thus causing great confusion in the nomenclature. A movement in the rock magma while cooling causes frequently a banded arrangement of the minerals, or flow structure." " Igneous rocks vary in color from the light gray, pink, and brown of the acid granites, syenites, and their volcanic equivalents (rhyolite, andesite, etc.) to the dark steel gray or black of the basic gabbro, peridotite, diabase, and basalt. The darker varieties are commonly called trap. This term is in very general use and is derived from trappa, Swedish for stair, because rocks of this kind on cooling frequently break into large tabular masses, rising one above the other like steps, as may be seen in the exposures of diabase on the west shore of the Hudson River from Jersey city to Haverstfaw.
"The sedimentary rocks as a class represent the con solidated products of former rock disintegration, as in case of sandstone, conglomerate, shale, etc., or they have been formed from an accumulation of organic remains chiefly of a calcareous nature, as is • true of limestone and dolomite. These fragmental or clastic materials have been transported by water and deposited mechanically in layers on sea or lake bottoms, producing a very characteristic bedded or stratified structure in many of the resulting rocks." "Metamorphic rocks are such as have been produced by the prolonged action of physical and chemical forces (heat, pressure, moisture, etc.) on both sedi mentary and igneous rocks alike. The foliated types (gneiss, schist, etc.) represent an advanced stage of metamorphism on a large scale (regional metamor phism), and the peculiar schistose or foliated structure is due to the more or less parallel arrangement of their mineral components. The nonfoliated types (quartzite, marble, slate, etc.) have resulted from the alteration of sedimentary rocks without materially affecting the structure and chemical composition of the original material." The rocks commonly used for road-building may be classified according to their popular designations as trap, granite, limestone, sandstone, and chert.
Trap. The term "trap" is commonly applied to most of the volcanic igneous rocks used for road-building, including basalt and diabase. While these rocks vary considerably in character, they are usually very com pact and tough, and may be classed as the best material for roads of heavy traffic.
" It is characteristic of all the trappean rocks that they have once been fluid from heat and while in that state have been injected into fissures of the rocks through which they have found their way toward the present surface of the country. Only in rare cases have they actually passed upward to the surface of the earth toward which they moved; their motion was arrested in the lower levels of the rocks to which the surface has been brought down by the agents of atmos pheric decay. The result of their consolidation under the conditions of pressure in which they cooled has caused these originally molten materials to be very compact, a state which is favored also by their chemical composition. This causes the materials to be very solid and elastic. They generally resist decay in such a manner that they often project above the surface, while the softer rocks on either side have been worn down."
Granite. The granites, including syenite and gneiss, vary widely in character and differ greatly in value as road materials. They may be classed as next in value to trap for wear in the road-surface, but are somewhat deficient in cementing properties.
* " In an examination of the bearing of the petro logidal characters upon the attrition results in this group three prominent factors stand out. They are: (t) Texture, (2) the kind of mineral, (3) the state of freshness of the minerals. With regard to the first of these it is evident that fineness and evenness of grain is an advantage, and that coarse grain or porphyritic structure is disadvantageous. It is on account of the granitic texture that the rocks of this group, taken as a whole, are not higher in the attrition scale.
"The influence of the kind of mineral (2) is not so easy to determine, but, other things being equal, a high proportion of hornblende appears to be favorable to resistance; quartz in a like manner is favorable because of its hardness and lack of cleavage.
"Fresh unaltered original minerals are not absolutely essential to a high capacity to resist abrasion; the two stones that take the best position in the test scale for this group are considerably altered — the feldspars are decomposed, and their substance is a mixture of smaller mineral units; the ferro-magnesian minerals have changed to chlorite and to fibrous uralitic hornblende." t "In the case of the igneous rocks it will be noted that the plutonic types with granitic granular structure (granite, syenite, diorite, and gabbro) are, as a rule, harder but inferior in toughness to their volcanic equivalents (rhyolite, basalt, and diabase). This is due to the more fully crystalline condition and coarser grain of the plutonic rocks. In the case of the volcanic types a compact crystal intergrowth and fine grain tend to increase the toughness rather than hardness of the material. The deleterious effect of atmospheric decomposition on rock texture is especially noticeable in the case of peridotite, andesite, and altered basalt, where the indifferent results of the physical tests, excepting cementing value, may be directly ascribed to the presence of such soft secondary minerals as kaolin, serpentine, calcite, chlorite, etc. . . . As has already been stated in a previous paragraph, the cementing value is as a rule found more highly developed in the igneous rocks which contain alteration products than in their unaltered varieties. This is especially true in the case of diabase and basalt, rocks very similar in origin and mineral composition. Con tinuing a step further, we note a marked decrease in toughness, hardness, and resistance to wear in the altered varieties of both these rock types over their fresher representatives. This is in line with what has already been said and indicates that the presence of secondary minerals in appreciable quantities, whether because of their softness or their indefinite semi crystalline condition, weakens the original mineral bond and tends to destroy the primary texture of the rock, while at the same time furnishing the elements for a high binding quality in the rock powder. Valuable results bearing on the decomposition of rock powders by water have been obtained by Dr. A. S. Cushman in a series of interesting experiments carried on in the chemical laboratory of this Office. Doctor Cushman has shown that hydrolysis takes place in case of many rock powders the moment they are wet, thus pro ducing secondary products (hydrated silicates) of a colloidal nature which greatly increase the binding power. This points finally to the conclusion that the mineral analysis of igneous rocks, besides providing a convenient means for comparison and classification, serves to a certain extent as a measure of their physi cal properties." Limestone. Limestones commonly possess the cementing power in fair degree, although lacking in hardness and resistance to abrasion. The cementing power has probably been commonly overestimated, because of the softness of the rock and the ease with which it usually packs in the road surface. Limestones are the most widely distributed and most generally used materials for road surfaces. They differ very widely in character, some forming an excellent material under moderate traffic and others being so soft as to offer little resistance to wear.