STRUCTURAL GEOLOGY.
The rocks of the crust, considered under the three headings of sedimentary, igneous, and metamorphic, present certain characteristic struc tural features. When in the form of flows the igneous rocks are arranged in layers, and they are often covered with beds of sedimentary strata.
Sheet-like intrusions of lava are also in beds. But dikes, bosses, and laccoliths are more irregu lar. These igneous rocks vary in texture, as has been already stated. Joint planes are com monly present, being due to contraction of the cooling masses which results in a breaking of the rocks. These joints at times assume almost mathematical regularity, as in the hexagonal col umnar jointing of Fingal's Cave and the Giant's Causeway. Many of the metamorphic rocks in herit sonic of the characteristics of the rock from which they were derived. But when highly meta morphosed they become massive and crystalline, resembling in this respect the igneous rocks.
However, owing to the influence of pressure, the metamorphosed rocks are characterized by a banding, often very marked. Veins are common in the metamorphic rocks, and the layers are often highly contorted under the strain of the tremendous pressure to which they have been subjected. Joint planes of later origin are also present. The sedimentary strata are character ized by arrangement in layers due to the assort ing action of the agencies which have caused their accumulation. This assortment is found both on a very small scale, represented by lami sometimes very complex and amounts to real contortion. A single fold, with a dip in but one direction, is called a monocline; the ordinary up arching of rocks is known as an anticline, and the downfolding as a syncline. These may be symmetrical or unsymmetrical, and in some cases me, and on a large scale, represented by changes in the nature of the material. For example, a series of shales, with many lamina;, may grade downward to a sandstone and upward to a lime stone. The minor variations represent the influ
ence of slight variations in the force or direction of currents or in the nature of material sup plied; the larger changes indicate more extensive changes, such as uplift or depression, which completely alter the conditions under which the sedimentation is taking place. A shallowing means coarser fragments, a deepening finer frag ments, because of change in the position of the coast-line. The sedimentary beds are lens-shaped because they die out in all directions; but the beds of coarser fragments, having less extent, are more lens-shaped than those made of finer frag ments.
The structure of sedimentary rocks often re veals the manner of origin. Coarseness indicates nearness to shore; limestone indicates abundant life; and the presence of currents, varying in velocity and direction, is indicated by cross, or current, bedding, in which the layers vary greatly in coarseness and in the direction and angle of inclination. This form of bedding is caused by river, wave, and wind currents. Ripple-marks, rain-prints, footprints of land animals, and mud cracks, formed by the cracking open of mud ex posed to the sun, are also commonly found, indi cating shallow-water origin for the deposits. From such evidence a remarkably large propor tion of the sedimentary beds are known to have been formed in shallow water.- While most of the rocks included in the sedimentary group are deposited as fragments, and hence are at first unconsolidated, the sedimentary strata of the land are mainly consolidated. This consolidation is usually the result of the deposit of some kind of cement by percolating water. Carbonate of lime, some sort of iron, and silica are the com mon cements. The presence of cementing mate rials in the ground water is illustrated by the replacement of woody matter by silica, forming petrified wood. At times this cementing mate rial gathers around centres, such as grains of sand or fossils, forming concretions.