Although a journey by way of the sink holes through a cavern system is to be commended for the reason that it is the course of the caverning waters, it is, on the whole, best to approach the cave through their exits along the banks of a stream or through the chance openings which are here and there made by the falling in of their roofs. One advantage of this cavity of entrance is that we can thus approach the cavern in times of heavy rain when the processes which lead to their construction are in full activity. Coming in this way to one of the domes formed beneath a sink hole, we may observe in rainy weather that the water falling down the deep shaft strikes the bottom with great force; in many of the Kentucky caves it falls from a greater height than Niagara. At such times the stones in the basin at the bottom of the shaft are vigorously whirled about, and in their motion they cut the rocks in the bottom of the basin--in fact, this cavity is a great pot hole, like those at the base of open-air cascades. It is now easy to interpret the general principles which determine the architecture of the cavern realm. When it first enters the earth all the work which the water does in the initial steps of cavern formation is effected by solution. As the crevice enlarges and deepens, the stream acquires velocity, and begins to use the bits of hard rock in boring. It works downward in this way by the mixed mechanical and chemical action until it encounters a hard layer. Then the water creeps horizontally through the soft stratum, doing most of its work by solution, until it finds a crevice in the floor through which it can excavate farther in the downward direction; so it goes on in the manner of steps until it burrows channels to the open stream. In time the vertical fall under the sink hole will cut through the hard layer, when the water, abandoning the first line of exit, will develop another at a lower level, and so in time it comes about that there may be several stories of the cave, the lowest being the last to be excavated. Of the total work thus done, only a small part is accomplished by the falling of the water, acting through the boring action of its tools, the bits of stone before mentioned; the principal part of the task is done by the solvent action of the carbonated waters on the limestone. In the system of caverns known as the Mammoth Cave, in Kentucky, the writer has estimated that at least nine tenths of the stone was removed in the state of solution.