In the lime-stone country there are many caverns of very considerable extent. The most noted is called Madison's Cave, and is on the north side of the Blue Ridge, near the intersection of the Rockingham and Augusta line with the south fork of the southern river of Shenandoah. It is in a hill of about two hundred feet perpendicular height, the ascent of which, on one side, is so steep that you may pitch a biscuit from its summit into the river which washes its base. The entrance of the cave is, in this side, about two-thirds of the way up. It extends into the earth about three hundred feet, branching into subordinate caverns, sometimes ascending a little, but more generally descending, and at length terminates, in two different places, at basins of water of unknown extent, and which I should judge to be nearly on a level with the water of the river; however, I do not think they are formed by refluent water from that, because they are never turbid; because they do not rise and fall in correspondence with that in times of flood or of drought; and because the water is always cool. It is probably one of the many reservoirs with which the interior parts of the earth are supposed to abound, and yield supplies to the fountains of water, distinguished from others only by being accessible. The vault of this cave is of solid lime-stone, from twenty to forty or fifty feet high; through which water is continually percolating. This, trickling down the sides of the cave, has incrusted them over in the form of elegant drapery; and dripping from the top of the vault generates on that and on the base below, stalactites of a conical form, some of which have met and formed massive columns. Another of these caves is near the north mountain, in the county of Frederic, on the lands of Mr. Zane. The entrance into this is on the top of an extensive ridge. You descend thirty or forty feet, as into a well, from whence the cave extends, nearly horizontally, four hundred feet into the earth, preserving a breadth of from twenty to fifty feet, and a height of from five to twelve feet. After entering this cave a few feet, the mercury, which in the open air was 50°, rose to 57° of Fahrenheit's thermometer, answering to 11° of Reaumur's, and it continued at that to the remotest parts of the cave. The uniform temperature of the cellars of the observatory of Paris, which are ninety feet deep, and of all subterraneous cavities of any depth, where no chemical agencies may be supposed to produce a factitious heat, has been found to be 10° of Reaumur, equal to 54½° of Fahrenheit. The temperature of the cave above mentioned so nearly corresponds with this, that the difference may be ascribed to a difference of instruments.