ROAD BUILDING ROCKS

Mineral Composition.1-Reports of geologists and mineralogists on road-building rocks classify them according to their origin as igneous, sedimentary and metamorphic.

Igneous rocks are those which have solidified from a very hot liquid condition and their physical condition, technically termed "structure," depends largely on the rate of cooling of the fused material. The "intrusive" or "plutonic" type of igneous rocks. cooled slowly at great depths below the earth's surface, and the minerals composing it are usually in large and well developed particles. This type includes granite, syenite, diorite, gabbro, and peridotite. The "extrusive" or "volcanic" types of igneous rocks cooled more rapidly upon the earth's surface and are finer grained. They frequently show a so-called "porphyritic" structure on account of the presence of larger crystals in a finegrained, dense mass forming the main mass of the rock. This type includes rhyolite, trachyte, andesite, basalt and diabase.

Sedimentary rocks are made up of fragments of minerals or shells that were moved about, mainly by water, and finally deposited on the beds of lakes or seas in more or less parallel layers. There they became cemented together by the pressure upon them and changes in the composition of a part of their constituents. This last change is of the same general nature as that occurring far more quickly in the case of plaster or mortar. This class includes calcareous types of rock like limestone and dolomite, and siliceous types like shale, sandstone, and chert (flint). Both types are usually distinctly bedded or stratified.

Metamorphic rocks were produced from the two classes just mentioned by pressure and heat. The long-continued shearing and compressive forces sometimes produced a "foliated" or "schistose" character, with a parallel arrangement of the minerals composing them, or a "massive" or "nonfoliated" character. Gneiss, schist and amphibolite are foliated metamorphic rocks, and slate, quartzite, eclogite and marble are nonfoliated metamorphic rocks.

1 Abridged from Bulletin 348, U. 8. Department of Agriculture, "Relation of Mineral Composition and Rock Structure to the Physical Properties of Rock Materials," by E. C. E. Lord, petrographer, Office of Public Roads and Rural Engineering.