FIG. 190.—Sketch map of the Oucane de Chabrières near Chorges in the High Alps, to illustrate the corrosion of limestone along two series of vertical joints (after Martel).] Within the hard rocks it is the sandstones which have the largest pore spaces, but in nearly all consolidated rocks there are additional spaces along certain of the bedding planes, the joint openings (Fig. 190), and the crushed zones of displacement, so that these parting planes become the trunk channels, so to speak, of the circulating water. It is along such crevices that in the course of time the mineral matter carried in solution by the water is deposited to produce the ore veins and the associated crystallized minerals. =The caverns of limestones.=—Where limestone formations have a nearly flat upper surface, a large part of the surface water enters the rock by way of the joint spaces, which it soon widens by solution into broad crevices with well-rounded shoulders. At joint intersections solution of the limestone is so favored that the water may here descend in a sort of vertical shaft until it meets a bedding plane extending laterally and offering more favorable conditions for corrosion. Its journey now begins in a lateral direction, and solution of the rock continuing, a tunnel may be etched out and extended until another joint is encountered which is favorable to its further descent into the formation. By this process on alternating shafts and galleries the water descends to near the surface of the water table by a series of steps, and is eventually discharged into the river system of the district (Fig. 191). Within the larger caverns the water at the lowest level usually flows as a subterranean river to emerge later into the light from beneath a rock arch. [Illustration: FIG. 191.—Diagram to show the relation of caverns in limestone to the river system of the district and to the “swallow holes” upon the surface.] From the plan of a system of connecting caverns it may often be observed that the galleries of the several levels are alike directed along two rectangular directions which indicate the master joint directions within the limestone formation. This is especially clear from the map of the galleries in the explored portions of the Mammoth Cave (Fig. 192).