The solvent acid of the water must be acquired in percolating through the several hundreds of feet of superincumbent earth and sandstone, as there is but one of these domes, "Sandstone Dome," that extends upward to the sandstone. The solvent water then, after finding its way into the vertical crevices of the limestone, gradually rounded them out like wells, until the pieces which occasionally fell from the top formed a sort of floor. Through the interstices of this floor, the dissolved substance of the rock is carried into some deeper and yet undiscovered cavities beneath. The floor itself gradually sinks, the domes grow higher, and the walls recede as long as the water continues to drip into them. It is not unreasonable to suppose that the substratum of limestone in all the country for many miles in the vicinity is perforated with these tremendous shafts; as those which are seen in the cave are only such as happened to be in the course of the tunnels composing the Mammoth Cave. It is not improbable that there are many others on every side, close to the line of the tunnels, but not yet connected with them. In any one of the above-mentioned departments, the description of one place answers for most others, except in dimensions. The following are a few out of many hundreds of measurements taken in as many different places:-- The "Rotunda," the first chamber from the entrance of the cave, is about one hundred feet high, and one hundred and seventy-five in diameter. The "Methodist Church," eighty feet in diameter, and forty in height. "Wright's Rotunda" is four hundred feet in its shortest diameter, being nearly circular; the roof seems perfectly level, and is about forty-five feet high. "Kinney's Arena" is a hundred feet in diameter, and is fifty feet high. "Proctor's Arcade" is one hundred feet in width and three quarters of a mile in length. The walls, which are about forty-five feet high, are nearly perpendicular throughout the whole length of the arcade, joining the roof nearly at right angles, and are so smooth that they look like hammer-dressed stone. "Silliman's Avenue" is a mile and a half in length and about forty feet in height, its width varying from twenty to two hundred feet. "Shelby's Dome" and the pit beneath it are two hundred and thirty-five feet in height, and about twenty-five in diameter. "Mammoth Dome" is two hundred and fifty feet high, and nearly one hundred in diameter. "Lucy's Dome," the highest in the cave, is sixty feet in diameter, and three hundred in height. Nine miles from the entrance of the cave is the "Maelström," a dry pit or well, one hundred and seventy-five feet deep, and about twenty in diameter; and from the bottom of this shaft may be seen the openings to three other avenues, which lead farther into this Plutonian labyrinth than mortal foot has ever trod.