The crystalline rocks are the least easily destroyed and are generally rough and jagged. The science of geology shows that limestones, sandstones and clays, were originally heaps of mud deposited, removed or arranged by water; and that boulders are transplanted masses from the parent rock, and are worn and rounded by mechanical attrition. Consideration of the manner in which rocks have been formed affords a fair indication of their stability in earthwork; for instance, many clay rocks are reduced to a pasty condition by the action of water and air, but with different results according to their nature, some requiring blasting to excavate them. On the other hand, there are sandstones which although soft in the quarry become hardened when exposed to the atmosphere. A dock cut in red sandstone when exposed to the atmosphere may slip and fail, but if the rock be protected from the weather or constantly covered with water it may be reliable.
As cuttings are near the surface and seldom at greater depths than 100 feet, it is hardly possible to know the angle at which a rock dips or whether there are faults and fissures in it, unless an examination is made upon the site, and this notwithstanding the geological character may be thoroughly understood. Local conditions may cause peculiarities which no law can determine, and although at considerable depths, deeper than railway or any works with the exception of well-sinking and mining are likely to reach, the nature of the earth is accurately ascertained; the surface soil may be in almost every conceivable variety, and also so dislocated, denuded and rearranged, that usually horizontal strata may be nearly vertical. Its character may be accurately known, but the lie of the surface beds or dip of the upper strata, or the order of supraposition cannot be invariably absolutely established; for instance, when crevasses, fissures and veins are frequent in earth upheaved and disintegrated by volcanic action, earthquakes and other disturbing causes, as in parts of South America, Japan, and other eastern countries, Italy, the Tyrol, Spain, &c., &c., slips of earthwork are to be expected, and the soil is likely to be much inclined, full of faults and probably water-bearing seams.
Under the comprehensive name of rock is usually included any earth from the hardest mass to be found to that which will crumble in the hand, as soft sandstone rock. The chief absorbent rocks with which engineers have to do are the limestones, sandstones, chalk, and clay rocks. Rock may also be simple or present the appearance of being homogeneous, or it may be a mass of different substances, be flat bedded, have open or close joints, and be what is called—
Solid rock.
Hard rock.
Dense, or compact, rock.
Loose rock.
Rock in loose layers.
Loose rock with cavities, caverns, and pot-holes of various earths.
Fissured rock.