_Movement along Joint Planes._--In some conglomerates the joints may be seen traversing the enclosed pebbles as well as the surrounding matrix; large blocks of hard quartz are cut through by them as sharply as if they had been sliced by a lapidary's machine. A similar phenomenon may be observed in flints as they lie embedded in the chalk, and the same joints may be traced continuously through many yards of rock. Such facts show that the agency to which the jointing of rocks was due must have operated with considerable force. Further indication of movement is supplied by the rubbed and striated surfaces of some joints. These surfaces, termed _slickensides_, have evidently been ground against each other. _Influence of Joints on Water-flow and Scenery._--Joints form natural paths for the passage downward and upward of subterranean water and have an important bearing upon water supply. Water obtained directly from highly jointed rock is more liable to become contaminated by surface impurities than that from a more compact rock through which it has had to soak its way; for this reason many limestones are objected to as sources of potable water. On exposed surfaces joints have great influence in determining the rate and type of weathering. They furnish an effective lodgment for surface water, which, frozen by lowering of temperature, expands into ice and wedges off blocks of the rock; and the more numerous the joints the more rapidly does the action proceed. As they serve, in conjunction with bedding, to divide stratified rocks into large quadrangular blocks, their effect on cliffs and other exposed places is seen in the splintered and dislocated aspect so familiar in mountain scenery. Not infrequently, by directing the initial activity of weathering agents, joints have been responsible for the course taken by large streams as well as for the type of scenery on their banks. In limestones, which succumb readily to the solvent action of water, the joints are liable to be gradually enlarged along the course of the underground waterflow until caves are formed of great size and intricacy. _Infilled Joints._--Joints which have been so enlarged by solution are sometimes filled again completely or partially by minerals brought thither in solution by the water traversing the rock; calcite, barytes and ores of lead and copper may be so deposited. In this way many valuable mineral veins have been formed. Widened joints may also be filled in by detritus from the surface, or, in deep-seated portions of the crust, by heated igneous rock, forced from below along the planes of least resistance. Occasionally even sedimentary rocks may be forced up joints from below, as in the case of the so-called "sandstone dykes." [Illustration: Joints in Limestone Quarry near Mallow, co. Cork. (G. V. Du Noyer.)]